Extending a house: the ultimate guide for building a house extension

Extending a house needn't be stressful if you go into the project fully prepared. This guide tells you everything you need to know, from costing and designing the extension to permitted development and building regulations

Extending a house: Kitchen extension by Ar'Chic
(Image credit: William Eckersley/Ar'Chic)

If you're planning on extending a house in the next few months, our guide to extending your home is here to help with every facet of the project. This three page feature covers everything from costing and designing extensions to permitted development and building regulations, you'll find everything you need to know about building an extension before you get started.

Our guide also covers additional extension costs that you might not have considered, interior fit-out costs, hiring professionals and design considerations – all so that you can get the best house extension possible.

Extending a house: budgeting for an extension

If you're extending a house, you can get a comprehensive extension cost calculation using our free extension cost calculator, but for a rough idea of what you might have to budget for your house extension costs, see below.

For planning and costing a single storey extension, expect to pay the following: 

Building work costs depend not only on where you live and the team you've hired, but on the results you're after. Expect to pay around £1,500 to £1,900 per square metre for an extension of basic quality; £1,900 to £2,200 per square metre will give you a finish of good quality; and £2,200 to £2,400 per square metre will allow you to achieve excellent quality.

In total, for an extension of 4m x 5m, the build cost would be around £30,000 and upwards for an extension of basic quality; you will pay from around £38,000 for a good quality build; and it will cost you from £44,000 for an extension with an excellent finish. 

These costs do not include professional fees (more on this below), which can cost around 10 to 15 per cent plus VAT. With these extra costs added, the lowest price you will pay for a 4m x 5m single storey extension would be just under £40,000 and up to £58,000 for an extension of excellent quality.

Extending a house: Side return extension exterior

(Image credit: Tim Mitchell)

For planning and costing a double storey extension, expect to pay:

Labour costs do vary around the country; these, along with the design quality, build spec and quality of materials will all affect the final price of your two storey extension. But as a rough idea, expect to pay per square metre for two-storey extension: £1,320 to £1,620 for basic quality; £1,620 to £1,860 for a good quality finish; and £1,860 to £2,100 for excellent quality. 

As with a single storey extension, the cost of fitting out your two storey extension will depend on the rooms within it – for example, a living space or bedroom will be cheaper to furnish than a new kitchen or bathroom.

‘As a very rough rule of thumb, costs should be split into 50 per cent for construction and 50 per cent for the interior,’ says Hugo Tugman, founder of Architect Your Home . If you do not want to compromise on the size of what you are building when extending a house, you may be able to make considerable savings on interior finishes.

It is important that you get quotes from at least four builders as it is almost certain that each of the builders will provide different costs for exactly the same work using the same materials. If you only approach one or two builders, you run the risk of only obtaining high quotes, rather than getting an idea of an average.

Extending a house: Sarah barker Brown and Nicolas Brown extension

(Image credit: Rick Mccullagh)

Real Homes Show | Find out more about the best ways to improve your home within your budget

If you are struggling with your space and wondering what kind of extension will work for you, or just what to do with your budget, check out the Real Homes Show. Every fortnight the Small Space Squad visit a homeowner trying to make their home work for them

Additional house extension costs to factor in

Architects’ fees for extending a house are usually between three and seven per cent of the total construction cost. Expect to pay around £2,700 each for planning drawings and construction drawings. Find architectural technologists on CIAT, and architects via Architects Register or RIBA.

Structural engineers' fees are £500 to £1,000 (if roof joists and foundations are specified).

Surveyors' fees – necessary if a survey of the existing house is required – total £500 to £1,500.

Planning fees in England are £206

A certificate of lawful development (recommended if extending under permitted development rights) costs £103. 

A request for discharging planning conditions costs £34.

Building control charges (vary according to your extension’s size) are between £200 (for an extension of 10m sq) to £900 (for 80 to 100m sq). 

A party wall agreement (if needed) is £700 to £1,000 per neighbour.

Project management costs also need to be factored in. Your building contractor or architect will charge 15 to 20 per cent – this on top of the net cost of labour, materials and overheads. Find out how you can save money if you project manage an extension or renovation yourself.

VAT at 20 per cent of the labour, materials and services. If your individual sub-contractors turn over less than the VAT threshold, they won’t charge you any VAT on labour, which can result in big cost savings.

Extending a house: bright kitchen-diner that opens onto the garden

Plastering or dry-lining and painting is around £85 per square metre.

Bi-fold or sliding doors cost between £1,500 and £2,000 per linear metre.

Additional fees for extending a house may include a tree report; a flood risk assessment (both £250 upwards); an ecology report (from £400); an archaeological report (perhaps several thousand pounds); a historic building report, likely if your home is listed.

Interior fit-out costs (see below).

What does an extension's interior fit out cost?

If you're extending a house it's really important to factor in all extension costs including the price of fitting out the interior right, from the beginning. The costs for the interior fit out of a single storey extension or two storey extension will largely be dictated by the room types you’re adding – a kitchen will be more expensive than a living room; adding an extra bathroom will be more costly than another bedroom or a home office, for example. You can add these figures to the ones above to give you an estimate.

The cost of adding heating will depend on all sorts of variables. Extending an existing central heating system may only need a few days’ work by a plumber, at around £150 per day (excluding materials). 

Underfloor heating will be more expensive, but is worth considering for a large, open-plan kitchen diner, if you are building an orangery or adding a conservatory. Electric underfloor heating is a cheaper installation choice, some elements of which you can do yourself, but will be more expensive to run than water-fed underfloor heating, which has a more expensive installation cost. You may also need a new boiler; expect to pay around £2,500.

See our individual guides to building a single storey extension and adding a double storey extension for more detailed breakdowns for each extension type.

Planning permission fees for extending a house

The planning fee for a residential single storey extension in England is £206. If you are extending a house under permitted development (see more below), you may want to apply for a certificate of lawful development, which will cost you £103; the fee for discharging planning conditions is £34 per request. 

Permitted development fees for extending a house

If you are planning to build an extension under permitted development rights, study the criteria carefully and apply for a Certificate of Lawful Development from your local authority. This only costs £86 and you will then have the paperwork in place to prove that your extension did not require planning permission.

Extending a house: planning >>