House Extension Cost 2023: Compare Building Prices UK

When your children are small, you don’t need much room. But, as they grow up, you need more space for them to spread out. So why not move house? Well, there are some very good reasons to opt for an extension and it doesn’t have to cost an arm and a leg: 

1. You have your home just as you want it. 
2. Your garden has matured, and that’s how you want it too. 
3. You and the kids have friends nearby. 
4. You like the school they go to. 
5. Your home isn’t too far from the workplace. 
6. Most of all, you like it where you live.

On top of this, you have the extortionate moving cost and upheaval, which seem to increase every month. So how much does a house extension cost compared to that of moving?

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The average cost of an extension in the UK

So, we’re considering the different types of extension you can build onto your house. You could easily extend just about any room (yes, even the basement). But, in the following article, we’ll look at a few common types in turn and you can decide one which one you want. 

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First, however, we’ll take one of the most common extensions. A single storey living space. We won’t look at the kitchen or bathroom just yet, because they need speciality work like plumbing and drainage. So, that leaves a living room or bedroom. 

Anyway, a typical single–storey medium–sized extension will cost about £1,000/m2 to £1,500/m2. If the extension is about 20m2 in area, the total cost will be between £25,000 and £37,500. 

Let’s consider a few more different extensions and the cost to build each of them. Remember, the cost will vary with many different factors. Such as the size of the extension, quality of fittings, the complexity of the internal and external design, where you live and the contractor’s day rate.  To accommodate some of these factors, we’ve calculated a price range based on a typical sized extension for that room.

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Here is a full list of UK extension prices:

On top of these prices, add 10% to 15% for professional fees, which include architect, planning permission, building regulations and structural engineer. Also, you must add VAT (20% at the time of writing). Finally, expect to pay about 20% to 30% more if you live in or around London.

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Extension cost factors

The cost of any extension will vary depending on several factors, irrespective of your choice of extension. 

1. The soil type on your property determines the foundations and how they’re constructed. 
2. The purpose of the extension. Rooms such as living–rooms and bedrooms are interchangeable. However, bathrooms and kitchens need specialised fittings that are very expensive. 
3. The amount and type of glazing will affect the costs. Simple double glazed windows will be standard. However, bifold doors, picture windows, orangeries and conservatories will have their unique fittings. 
4. The size of the extension affects the price per square metre. Most of the costs go into foundations and roof, the two most important part of any building and these will be in place, no matter how many storeys the building has. 
5. The location of your home will also determine the cost of an extension. Unless you live on a remote island and need all the materials specially shipped, it’s mainly labour costs that vary with location. Furthermore, in London and the surrounding areas, you‘ll find that labour prices are around 25% more than in other locations.  
6. Do you need a project manager? It’s important to have someone to organise the trades and the materials. If you have DIY experience, then it’s possible to manage your project. But, you’ll have to devote a lot of time to it and know a heap of stuff about construction and the UK Building Regulations. Generally, it’s better to hire a project manager if it’s a large and complex project. 
7. Building Regulations and Planning Permission, both enforced by the local council, determine the construction method, what type of materials you can use, and what you’re allowed to build. 

Single storey side and rear extension 

This is a typical extension that the majority of people choose. It’s essentially, a ground floor room built onto the side of your house. Typical prices range from about £1,000 to £2,000/m2, depending on the quality of fittings and materials. A single–storey extension is a basic type. And, this will be the starting point before you add on specialist fittings for kitchen, bathroom, utility room and so on.

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Double-storey extension 

A double-storey extension is very similar to a single storey to build. You just have more floors! So, you’ll have to buy more materials, that should be obvious. But, both a single and a double both need a roof and foundations, so there shouldn’t be any change there. Because of this, you won’t double the price for building two storeys. In general, the pricing will be 1.5 to 1.75 times the cost of a single storey. However, there are some important things to consider. Firstly, ensure the foundations can carry the extra load from the additional storey. And, remember that upstairs windows must provide an adequate fire escape.  

Kitchen extension 

Once again, this can be regarded as a single storey extension with extras. The room will be the same, but you must have drainage, hot and cold water, ventilation for an extractor fan, mains gas, and additional heavy–duty electricity supply for a cooker. Moreover, most modern kitchens require storage cupboards, worktop counters and built–in appliances. To allow for these, add on an extra £10,000 or so, depending on the quality of kitchen units and appliances. 

Bathroom extension 

This will be similar to a kitchen extension. If you build it on the ground floor then it’s part of a single–storey extension. But, if you decide on a double-storey, you’ll probably have your bathroom upstairs. A bathroom extension, like a kitchen, will need specialist fittings, such as shower, bathtub, washbasin and toilet pan. You might also incorporate an airing cupboard and new central heating boiler. So, extras include the bathroom suite as well as plumbing, drainage, and new boiler. Expect to pay an additional £5,000 or so on top of the standard extension price, depending on what appliances you incorporate into the bathroom and their quality.

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Glass box extension 

You might not have heard of this before. But, a glass box extension is exactly what it sounds like. You could consider it to be similar to a conservatory but with no metal supports anywhere. A typical ‘glass box’ uses structural glass units, with glass beams and fins. Remember, a  ‘glass box’ extension differs from a conservatory because it’s part of the house and within the thermal envelope of the property. Whereas, a conservatory is separated from the rest of the house by an external door. Therefore, you can’t use any old glass. The specifications for the extension must comply with the Building Regulations for heating, ventilation, and thermal performance, as well as being structurally sound. 

Orangery extension 

An orangery is a cross between a conservatory and a standard room. They tend to be more expensive than an equivalent sized conservatory because they are part of the thermal envelope of the building, incorporate a proper roof made from timber slate and tiles, contain far more traditional building materials such as brick and timber than a conservatory does. And, because of the extra weight, they need foundations of the same quality as a traditionally built extension. Also, orangeries traditionally contain a roof lantern. This is a central portion of the traditional roof, built from glass to allow extra daylight into the living area. 

Conservatory extension 

Of course, you can buy these as large or as small as you want. Roof profiles differ as well. You can have a standard lean-to roof, a Georgian, Victorian, Edwardian, or even a combination of these. Some conservatories have glass walls from floor to roof, while others have a dwarf wall of masonry about 1m high topped with glass windows to the roof. The cheaper conservatories range from about £5,000 including installation, up to £40,000.  However, you can see the average cost in the table.  

Typically, conservatory glass will be clear. However, if the windows reach the ground, the lower half will often have frosted or some other opaque pattern. Moving up to the roof, cheaper models tend to have clear double–glazed polycarbonate sheets as a roofing material, while more expensive models might use toughened glass. If you choose a glass roof on a conservatory designed for polycarbonate, remember that you’ll need extra supports and stronger foundations as glass weighs much more than plastic. 

Utility room extension 

You probably already know that a utility room is somewhere to keep the washing machine and tumble drier, have a few wall units and a sink. If you have a dog, then it’s a good place to keep its bed. You can also keep outdoor coats and boots there, without bringing them into the house.  Also, you’ll need plumbing and drainage in the utility room.  But, if the room is next to the kitchen, it’s no problem for a plumber to bring the pipework through the wall.  

Some people might use a cheap conservatory as a utility room, while others use a lean-to porch. Many people like the idea of covering in the outdoor space between the kitchen door and the garage to make a special place for laundry. But, if you want to do it properly, the cost will be at least £10,000, rising to more depending on the floor area and the type of roof you choose.

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Multistorey extension 

In theory, there isn’t any reason why you can’t build an extension with more than two storeys. However, in practice, you’ll find that the local planning authority will only let you do this in exceptional circumstances. The main drawback is that you aren’t allowed to build higher than the roof of the existing property, which in most properties is two storeys. This means you can only build a multistorey extension if you live on a slope and build into the ground.  

Other planning regulations might also make it difficult to build a multistorey. Examples include, you can’t build more than 3m from the rear wall of the existing house or nearer than 7m to the boundary opposite the rear wall. Suffice it to say, that before you start planning a multistorey, contact the local planning department and ask their opinion. If you live on a housing estate with neighbours close by, expect your enquiry to be politely refused. 

Wrap-around extension 

This type of project extends two sides of the existing house, meeting at a corner. This is a major project that needs many structural alterations. Usually, a householder might choose this type if their living room is too small and needs to be larger in width and length. Remember, you’ll be constrained by how far you’re allowed to build up to your boundary, and by how much of your land your extension will cover (No more than 50% of the land outside your original house).  

Garage extension 

Modern garages built by a developer seem to be just large enough to park a small car. If you want to buy a larger family car or keep other things in the garage, such as bicycles or a workbench, you’ll either have to build a garage extension or park the car on the road.  

The UK has standard sizes for new build garages: 

• Small single garages measure 2.4m x 4.9m. 
• Medium–sized single garages measure 2.7m x 5.5m. 
• Large single garages measure 3m x 6.1m. 

If a typical standard family car measures about 4.6m x 1.8m, you can see that even with a large garage, you must park to one side to allow room to squeeze out of the driver’s door.  

The solution to this problem is to build a garage extension to your specifications. There are many different designs available to choose from. And, you can pretty much build one to the size you want. However, remember, you won’t usually need to ask for planning permission as long as you comply with the restrictions placed on “Permitted Developments”. Contact your local council’s planning department for advice on what’s acceptable in your area.

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Extension Cost Factors

So far we’ve talked about the cost of building only. But, there’s far more to extending your property than that. Below, we’ll take a look at what other things you have to consider and how much each one costs. Remember, all prices exclude VAT and will vary depending on where you live. 

The following sections describe the function of each item mentioned in the previous table. Not all the items refer to every extension build. And, not all the trades and the corresponding Building Regulations will exercise their full range of responsibilities.  

Structural engineer 

The size of the extension determines how much work the structural engineer has to do. Initially, he must calculate the total load on the foundations and list thickness of concrete, depth of foundations, and the reinforcing necessary for the concrete. Extensions often involve removing existing walls, and these must be replaced by suitable lintels supported by brickwork piers. At all times, the structural engineer must comply with the UK Building Regulations for complete safety. He must also provide a report detailing the calculations for use by the architect and the Building Control department. 

Surveyor 

The surveyor establishes the location of the property and its extension. Initially, they work from established Ordnance Survey points of known coordinates and altitude. Working back from these known points, they determine the exact position and elevation of the property relative to other properties. Surveyors can draw their own plans for submission to the planning department or give the data to the architect for including in theirs. Levels and dimensions will be transferred to the construction drawings for the builder to work from. 

Architect 

The architect takes data from the structural engineer and surveyor and draws plans suitable for a planning application. He will also produce construction drawings for the builder and other trades to work from. The architect will liaise with the local authority during the planning application. And, when the construction phase begins, he might act as a project manager if the project doesn’t have a separate one. Or, he will liaise with the project manager to ensure his instructions are carried out. The architect usually represents the client in discussions with the local authority and the building trades. 

Planning fee 

The client pays the local authority for the admin costs involved in Planning permission and Building Regulations. This must be paid when the architect submits the plans.  

Planning permission ensures the proposed development 

1. Complies with the local authority’s plan for the area. 
2. Ensures that the development doesn’t affect the neighbours’ rights in law. 

The Building Regulations specify a minimum standard for design, materials, workmanship of almost every part of the development, but especially those that impinge on safety. 

Tree report 

A tree survey takes place on public and private land to give the planning authorities useful information on which they can make their planning decisions. A professional arborist carries out the survey using the instructions specified in BS5837: 2012, Trees in Relation to Design, Demolition and Construction.   

Key pieces of information about the tree include: 

• Species of the tree. 
• Height and diameter of the tree. 
• Overall health of the tree. 
• The estimated age of the tree. 
• Estimated life expectancy. 

From this information, the arborist recommends what can be done to the tree. The developer or householder wants to know whether they can remove the tree. Whereas, the planning department wants to know whether removing the tree will contravene any legal or ecological requirements. Even if the tree is a protected species, the report might recommend that it’s removed because poor health and general condition makes it unsafe. Alternatively, the report might specify the tree should remain. In this case, the landscape designer or architect will take this into account when designing the property and garden.

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Flood risk assessment 

Certain planning applications need to contain a flood risk assessment of the proposed site. This tells the applicant and the planning department how likely it is for the site to be flooded. Usually, you would pay a flood risk specialist (often a surveyor) to do this work. Although, for most applications, it is a simple task that most people can do themselves. If you need advice on how to do this contact the Environment Agency. 

Ecology report 

This survey provides expert advice to the planning department on the ecological impacts of the development. Small developments, the category that an extension falls into, usually requires a Phase 1 ecological survey. This involves a site visit followed by a report to be submitted with the planning application. If you aren’t sure whether you need an ecology survey, there’s a free online tool to help you decide whether to call in an expert. 

Archaeological report 

An archaeological report’s purpose is to alert the planning authority if the area is of archaeological importance. This doesn’t mean that you won’t be able to develop any land in this area, It’s purely to let the planning department know that the area exists and they’re made aware of any operations that might disturb the ground, cause flooding, or cause tipping. 

The general builder or bricklayer 

This person will be responsible for levelling and clearing the site before work commences. Also, the foundation trenches need to be dug. Followed by, inserting sewer and land drain pipes, and pouring concrete foundations.  Some buildings have a concrete slab ground floor too, and that will also be in his remit. Once the concrete has dried, the bricklayer builds external and internal brick or concrete block walls.

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Carpenter 

The carpenter’s responsibility is anything to do with woodwork. That means, wooden door and window frames, timber framing, floor and ceiling joists together with structural roof rafters. He will also be responsible for hanging internal doors, installing stairs, and fitting skirting boards. Carpenters’ responsibilities fall into two types: 

• First–fix. This is all the woodwork done before plastering. Examples include structural timber, floorboards, stairs, plasterboard walls and ceilings, and door frames. 

• Second–fix. This is woodwork after plastering. Examples include skirting boards, architrave, hanging internal doors, fitting kitchens and boxing around pipework. 

Plasterer 

A plasterer covers blockwork and plasterboard indoors to provide a finished surface on walls and ceiling. He might install plasterboard too, but more often that’s left to the carpenter. Exterior plastering is called rendering and is a cement mortar spread onto external walls for weatherproofing. Plasterers sometimes also lay a cement screed onto concrete floors to provide a smooth level surface suitable for carpet or vinyl floor covering. Plastering comes under the Building Regulations covering soundproofing, heat insulation and fire proofing. 

Plumber 

A plumber installs all the water and gas pipes during first fix (before plastering, ceiling and wall plasterboard and floorboards). During the second fix, he or she installs sinks, bathroom suites, radiators and water heaters. Some plumbers also install lead flashing on roofs.  

Plumbers must be qualified for work according to the Part H of the Building Regulations for drainage, Part G for Sanitation, hot water safety and water efficiency,  UK Water Supply (Water Fittings) Regulations, and Part J Combustion appliances and fuel storage systems (for gas and oil heating). 

Electrician 

An electrician also has a first and second fix. Once again, the first–fix is before plastering. Usually, this includes installing electric cables behind plasterboard walls and ceilings and under floorboards. Second–fix electrical work involves installing light fittings and switches, power points, electric shower connections and a consumer unit with circuit breakers. Electricians work under very strict regulations. Part ‘P’ of the building regulations covers all domestic electrical work and must be checked by the Building Control department or be self–certified by a ‘Part P’ electrician. 

Roofer 

A roofer covers the structural roof timber with waterproof and breathable fabric and installs roofing slates or tiles. A roofer might also install leadwork for waterproofing. Usually, a roofer will also install guttering and other rainwater goods. A roofer usually does this work as soon as possible so that the interior of the structure is dry and internal work can carry on unhindered by bad weather. Without a finished roof, external doors and windows, a building will be open to the elements and other work will stop until this is completed. Roofers must work according to the Building Regulations too. 

Painter and decorator 

Usually, this is the last person in the house before handover to the customer. Their responsibility is to finish the surfaces, where necessary, to give an attractive surface finish on the interior and a waterproof finish on the exterior.

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UK Planning Permission & Building Regulation 

In addition to the instances covered in the previous sections, designing and building a house extension will be governed by other Planning Permission and Building Regulations. 

Planning permission 

Any external building work will come under the planning regulations governing the appropriate type of construction. However, some types of buildings are exempt from planning permission as long as they follow certain rules. These exemptions are known as ‘Permitted Development’ rights, and as long as you follow the rules, you can pretty much build what you like. A full range of Permitted Developments will vary depending on where you live in the country and the rules laid down by the local authority. These are available online or contact your local council for advice. 

However, certain restrictions are common to all extensions. These include: 

• An extension (and other outbuildings) can only cover a total of less than half the area of the land around the original house. So, under permitted development rights, this is the maximum size of extension you can build without planning permission. 
• Extensions must not be higher than the highest part of the existing roof. 
• The eaves on the extension must not be higher than the eaves on the existing house. 

• If the extension comes within 2m of a boundary, the height of the eaves cannot be higher than 3m. 
• Extensions must not be forward of the ‘principal elevation’s’ building line. 

If the extension extends beyond the side elevation of the existing house. The extended building  

• Can’t exceed 4m high. 
• Must be a single storey. 
• Must not be longer than half the width of the existing house. 

Note that any extension within a conservation area must have planning permission too.

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Building regulations 

As you would expect, every extension to an existing property needs to comply with Building Regulations. 

These categories include: 

• Foundations. 
• External walls. 
• Internal walls. 
• Roof. 
• Drainage. 
• Electrics. 
• Kitchens and bathrooms. 

Also, some work might affect your neighbours. You must, therefore, find out whether your extension has restrictions imposed by The Party Wall Act 1996.  

House Extension Cost Q&A 

Can you extend the front of the house? 

Under permitted development rules, you aren’t usually allowed to extend to the front of your house. Exceptions include where the front of the house is a substantial distance from a public road. Check with your local planning department for clarification. 

How much do foundations cost for an extension? 

Unfortunately, the answer to this question is almost impossible to answer without breaking ground and seeing what the excavation looks like. There are so many variables. Such as,  

• Type of soil. 
• Depth to the subsoil. 
• Height of water table. 
• The size and weight of the extension. 
• The type of foundation. 

However, the average cost for foundations will be between £100 and £130/m2. This includes excavation, removal of soil and cost of ready–mix concrete. 

Can I build my own extension? 

You can do this. But…. You will need to either teach yourself many trades from scratch, find out a lot of things about planning law and so on. All this knowledge takes many different people years to accumulate. Tradesmen follow apprenticeships and have many years of experience perfecting their trade. Architects, surveyors, and structural engineers spend years at university learning their skills too, before gaining experience. Plus, the planning authority and building control department will need to know that you’re capable of taking on the roles of the “difficult” trades like electrician, and plumber, as well as the easier ones. So, the answer to your question will be that in our experience, you can do certain jobs that don’t require much skill. But, leave the difficult bits to the experts. That’s what they’re good at.

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Is it cheaper to extend or rebuild? 

You probably won’t believe this. But, it’s generally cheaper to demolish and rebuild, than to build a major extension and renovate the rest of the house. However, having said that. If you only want to build a small extension to your kitchen, bathroom or some other room, it would be silly to knock the entire house down. If you aren’t sure, ask a professional such as an architect who can give you much better advice based on the existing condition of your home and how you want to improve it.  

Get Extension Quotes

Often the cost of building an extension makes this an attractive proposition compared to moving house. So, if you think you need extra room look at all your options first before taking the plunge. As a first step, complete the form on this page to get 3 or 4 quotes for building an extension onto your existing property. Then you can see for yourself how much the costs compare.

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