House Extension Cost & Prices 2021

House extensions are pretty much the holy grail of home improvements, adding space and value to any home. Most homes can be extended in some way, offering a very simple means to quickly add space to a home without moving. 

There are many different types of home extensions and prices vary hugely depending on location and the scope of the work. Virtually any extension will contribute some value to a home.

In many parts of the UK, extensions make very good economic sense and rank as a strong investment for the home. 

This article will review the cost of building different types of house extensions, factors that affect the cost and other common questions.

How Much Value Do House Extensions Add to a Home?

House extensions will always add some value to a home, provided the quality matches the quality of the overall building and interior finishes and fittings are also good quality.

The rule of thumb for what value extensions add to a home is typically around 5% to 10% of the home’s value, but this can vary wildly. For example, a 25m2 extension in Kensington and Chelsea might be worth over £500,000, according to the ONS (which might well be 10% of the home’s value). 

In areas where space is at a premium and house prices are rising, extensions can be worth much more than 10%. Nationwide found that 3-bed semis with extensions can be worth as much as 23% more, for example. Extensions are also nearly always better value than upsizing into a home with one or two rooms more. 

In fact, the ONS offer a tool here for checking how much value an extension can add to a home. 

In short, extensions offer excellent financial value both in the medium and long-term and are likely to pay for themselves in the vast majority of situations. Of course, you have to factor in the use of the space too. 

Types of Extensions 

There are 5 main types of extensions: 

  • Rear extensions 
  • Side return extensions 
  • Wrap-around extensions
  • Double-storey extensions 
  • Roof, garage and basement extensions 

Rear Extensions

Rear extensions extend out of the rear of the house. They usually extend out onto the garden and/or patio. Rear extensions are very simple if the garden is flat, but if the garden slopes or is otherwise awkward to extend out onto, they can be pretty complex. 

Rear extensions can create a platform for a two-story extension on top of the rear extension. Popular for open plan living areas and kitchens. Any large or bi-folding doors will come at a considerable extra cost. Consider also an orangery, which could produce similar results. 

  • Makes use of rear space, either patio or gardens
  • Straightforward for flat gardens 
  • Can create a large open place space
  • Potential for extending upwards too 

Side Return Extensions

Some houses have alleyways between either the house and the garage or the house and a neighbour’s house. There might otherwise be disused space or side-channel access on one side of the house that can be extended into. 

Side return extensions make use of this alleyway space and are typically some of the cheapest extensions. One major consideration is that the extension might directly border a neighbour’s wall which concerns the Party Wall Act, though this is not usually an issue. 

  • Makes use of disused side passageways and alleyways 
  • Can be a cheap and easy form of extension 
  • Good for extending kitchens and living rooms 
  • Ideal for homes with alleyways between the house and garage 

Wrap Around Extensions

Wrap-around extensions combine the above extensions, extending both out into a side space and to the rear of the house. A popular large downstairs extension option. 

  • Combines the above two extensions 
  • Creates a large amount of space
  • Can be divided into new rooms
  • Ideal for creating large open place spaces

Double-Storey Extension 

Double storied extensions simply extend upwards, creating another floor. They can be added to existing or new parts of the building and create upstairs space. Double story, or upstairs extensions, create extra bedrooms and upstairs floor space. Since they tie into the downstairs’ foundations, they’re often not as expensive as they might seem. 

  • Creates upstairs floor space
  • Can be built on top of the existing building or on top of new extensions 
  • Can be built on top of garages 
  • More likely to need Planning Permission, more on this shortly 

Loft, Garage or Basement Extensions

All of these extensions convert existing interior spaces rather than building new spaces. They’re amongst the cheapest options. Can be combined with other types of extensions to create a vast amount of new space. 

  • Makes use of existing rooms 
  • Cheap 
  • Combinable with other types of extensions 
  • Subject to Building Regulations to ensure the space can be lived in

Planning Permission for Extensions

House extensions will not always require Planning Permission and may come under Permitted Development. Permitted Development grants homeowners rights to extend their property without Planning Permission, subject to certain conditions. 

Note, limitations apply to homes in: 

  • Conservation Areas
  • Areas of Outstanding Natural Beauty
  • World Heritage Sites
  • National Parks
  • Listed buildings

First and foremost, Permitted Development applies only to the ‘original fabric of the building’, so if the home has already been extended then you don’t have the right to extend even more under PDRs. 

For all extensions:

  • Only half the land can be covered by extensions
  • Cannot be higher than the original roof or 3m within 2m of a boundary 
  • Can’t alter the roof 
  • Must not extend beyond the front of the house

For side extensions: 

  • Cannot exceed 4m
  • Single storey only
  • Only up to half with the width of the original house 

For single-storey extensions:

  • Cannot extend beyond the rear wall by more than 4m for detached or 3m otherwise 
  • Cannot exceed 4m in height

For more than one storey: 

  • Must not extend beyond the rear wall by more than 3m 
  • Roof pitch must try to match original roof

Following a special consultation scheme with neighbours (not Planning Permission), It’s possible to extend by: 

  • Over 4and up to 8 metres for detached houses
  • Over 3 and up to 6 metres for all other houses

Always contact your Planning Authority if you’re unsure. Your architect and builder will also be able to tell you if your plans require Planning Permission. 

Building Regulations

Extensions will also need to comply with Building Regulations which specify everything from fire safety to ventilation. Your contractor will talk you through the Building Regulations process, as the extension may need to be checked periodically throughout construction. 

Average Cost of Extensions in the UK

By analysing 10 primary and 12 secondary sources, we found the following averages for house extensions in the UK. Prices just include the extension, including all structural costs, electricity, plumbing and plastering. Prices do not include bathrooms or other plumbing costs. 

Type of ExtensionRear or Other Single-Storey Side Return or Wrap-AroundDouble-Storey
Budget (Per m²)£1,250 - £1,800£1,250 - £1,600£1,250 - £1,800
Mid-Range (Per m²)£1,350 - £2,000£1,350 - £1,800£1,350 - £2,000
High-End (per m²)£1,750 - £3,000£1,750 - £3,000£1,750 - £3,000
Example 25m² All-in-Cost£43,750 - £90,000£43,750 - £90,000£43,750 - £90,000

Extra House Extension Costs

The main extra costs of house extensions are plumbing costs, in the case of fitting taps, toilets, showers or entire bathrooms, and the costs of fitting a kitchen. Costs for kitchens and bathrooms vary with the quality and appliances required, e.g. a tap and toilet won’t cost nearly as much as a full en-suite bathroom for a high-end extended bedroom. 

Interior design costs also vary, with high-end designer fees costing more than standard. High-end wallpapers, light fittings, etc, will cost vastly more than standard fittings. 

Party Wall Agreement fees may also cost some £1,000 to £2,000 and Planning Permission, if applicable, will cost around £200. 

 Mid-RangeHigh-End
Bathroom Cost+ £2,500 - £5,000 to total cost + £5,000 - £15,000 to total cost 
Kitchen Cost+ £10,000 - £25,000 to total cost + £12,000 - £50,000 to total cost 
Bifold Doors+ £5,000 to £8,000 to total cost + £7,000 to £12,000 to total cost 
Interior Design+ £2,000 to £8,000 to total cost + £5,000 to £10,000 to total cost  
Party Wall Agreement £1,000 to £2,000
Planning Permission Around £200

Factors Affecting the Cost of a House Extension

There are many factors affecting the cost of a house extension. This is a breakdown of the various costs associated with house extensions. 

Design and Planning 

Design and planning will involve an architect and structural engineer, and possibly also a surveyor. Plans will have to be drawn up, submitted to the Planning Authority if applicable, and budgets created with estimated costs.

Beware that quotes are different to estimates. Estimates are a much looser prediction of the cost, whereas a quote will actually break down and itemise the costs into something a lot more concrete. 

Type and Format of Extension 

The type and format of the extension has a massive bearing on the overall cost. We’ve already covered some of the differences between different extensions. The need for reinforcing foundations is a major one here.

Also, since the extension will need to try and match your existing home’s materials, higher-end materials (e.g. flint exteriors) will cost more. 

Groundwork 

For extensions over garden areas, groundworking will be needed to prepare the ground for foundations. Drainage, digging and levelling will all cost and costs will be considerably higher for uneven terrain. 

Building Site

Easily accessible sites are cheaper to work on. Moving various drainage, pipework or other fixtures will incur additional costs (and could be awkward if they’re owned by neighbours or the council). 

Windows and Doors

Large high-end windows and doors can add considerable cost to any extension, particularly large rear extensions. Bi-folding doors can add £5,000+ to the cost of a rear extension. 

Fixtures and Fittings 

As briefly mentioned, fittings and fixtures such as lighting, wallpaper, paint and other interior design features can add considerable cost to any extension, especially if you’re hiring separate interior designers. 

How Long Do House Extensions Take?

Extensions rarely take less than 2 months, even for simple jobs. 3 to 6 months is probably the fairest rough estimate. This depends on the complexity of the job, access to the site and also the weather. 

How to Choose a House Extension Contractor 

Choose a reputable builder with a strong portfolio of ongoing work. Always double check project references. Choosing a builder who is part of the Federation of Master Builders (FMB) guarantees a builder who has been independently vetted.

RICs suggested always waiting for a reputable builder rather than rushing to find one who is available. 

Make agreements based around: 

  • The provision and agreement of full itemised quotes 
  • The start of works and expected duration of the job
  • Provisions for working hours
  • Waste disposal and noise 
  • Possibility of independent arbitration in the event of a dispute

Get Prices on Your House Extension

We’ve done our best to give you a good idea of what you can expect to pay for a house extension.

However, our guides are not a substitute for a fixed quote specifically for you.

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