What’re the Costs of Building an Extension?

Architecture drawings and a mini house

So, how much do they cost?


It’s difficult sometimes to get a clear understanding of the processes, costs and regulations when it comes to extensions. That’s why at Planning2Build UK we are a great place to make your home renovations easier for you. We can help you understand all of the processes and give you a better understanding of how much an extension can cost before you dive into something unknown.

The cost of extensions varies in price depending on what you would like done, the smaller and less complex extensions will be cheaper. Other factors such as where you live can alter the prices, for example, if you live anywhere near London it’s likely your extension will cost more. So, bear this in mind, depending on where you live prices can fluctuate.

What type of extensions are best suited for me?


To give you an idea of how many extensions are, below we have organised different types of extensions, prices and important things to remember along the way.

  • Single-storey extension? An estimation for the cost of a single-storey extension is normally between £1,500 to £2k per m2 based on 2019 reports. But if you are in areas like London these are places of high value, this could be between £2,500 to £3k + per m2. At these prices they will be good quality extensions, the basic quality will cost less. For example, if you are looking for a small extension 5m x 5m is would be around £32k, but in London areas, the price could be over £50k. Also, remember that’s just the extension, you will need to add 10-15% for fees from architects, planning permission application, building regulations, structural engineer, VAT, etc. These will all be added to your total bill. The total for the extension with the added extras could cost £40k, but in higher value places could be up to 70k. If you have a bungalow they are on the same level, the extensions will cost similar to a single story extension, depending on where you live.
  • Two-storey extension? If both storeys are the same size, the rule is to add 50% more to the build cost of a single-storey extension. £40k + 50% = £60k, plus 10% for professional fees and VAT. Again higher value areas will be more money.
  • Kitchen or bathroom extension? When you start thinking about these areas it does get more complicated. Permissions and plans will have to be put in place for plumbing to be installed by a plumber. You need to express your expectations so it’s done properly to your expectations. An average bathroom can cost about £5k, but this depends on your finishes and if you are installing a new bathroom suite (e.g., toilet, shower, bath). A Kitchen will be around £10k, this figure is based on a low/mid-range kitchen.
  • Converting your attic/basement? While there are no minimum ceiling height regulations in Building regulations, there is a minimum height for practical purposes. Take note that attic room and basement conversions need a floor-to-ceiling height of 2.1m minimum, although most are 2.4m.
  • Finishings and fittings? As we have mentioned before about kitchen and bathroom extensions and extras will add up to your bill. If you’re happy with simple painted walls, standard carpet or wood floors, lightings and electronics that will keep your costs down. But if you want higher-end stuff, tiling, fitted joinery, wooden flooring this will be higher. Make sure you add these extras into your books when calculating how much your extension will cost you if they are simple or luxurious.
  • What about the windows? Remember every detail, most people forget about the windows. These can be an expensive addition to your build, of course, the bigger the window the higher the cost. Also, there are higher levels of glazing, side doors, and enhanced insulation will all add to the cost. Think carefully about any windows before you draw up any plans, don’t be too ambitious if you can’t afford it.

Other things to think about:

  • Trees – Be careful when planning to extend out your home as you may have trees in the way and many of these trees are protected by Tree Preservation Orders. So, make sure you take trees into account when requesting planning permission. If you remove or alter a tree without the correct permission, you could end up paying a large fine.
  • Building site constraints – Remember to add in additional costs about your site, for example, if you have a complicated build on your site you need to ask your builder to add in the extra costs, so you both know what you are paying for.
  • Quotes – look for at least 3 or more quotes for the same contractor from different builders to ensure quotes are fair. You will be able to compare contractors from the quotes you will get to see if they are accurate and fair quotes and evaluate the prices, timescale and how much experience they have.

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