If you wanted, you could look up the average price of building an extension in the UK and get an answer from Google. But that probably wouldn’t tell you an awful lot about your particular situation. Sure, you’d get a ballpark figure, but not necessarily something that would give you a target figure based on your specifications and needs.

Many architects will suggest around £1,750 per square metre for an extension. This would usually be a cookie cutter design as many homes in London have a similar layout therefore a simple design can be achieved for this.

We don’t take this approach. We design specifically for you.

Read about Cost of House Extensions
For a single-storey extension – be it a side-return extension or rear house extension – we allow between £2,500 and £3,000 per square metre, depending on finishes. These figures can also apply to renovating the existing areas within a house.

These are, however, just averages. Let’s delve into the details.

What Affects The Cost Of A Build?

The cost of building an extension varies according to several factors:

The Location Of Your Build

In some areas, acquiring building materials and labour is more expensive than others. Labour, for instance, is cheaper in the north and Wales than it is in London. Similarly, delivering materials to well-connected cities like Birmingham is often less expensive than doing the same for the Outer Hebrides.

Whether You Choose One or Two Stories

Two-story extensions are a more significant engineering challenge than single-story builds and typically cost more than single story constructions. How much more money they require, again, depends on other factors concerning your build. But usually, builders recommend that you add 50 per cent to the cost of a single story extension.

Say, for instance, that the cost of a single-story extension with a similar footprint is £100,000. Add 50 per cent to that total, and you wind up with £150,000 in total expenses for double story equivalent.

From the maths, you can see that double story extensions are more expensive overall but offer higher value for money per square metre. If a 20 square-metre extension costs £50,000, then the price per square metre is £3,000. A 40 square metre £75,000 double-story extension on the same footprint is £1,875 per square metre – 25 per cent less.

The Quality Of The Build

Clearly, entry-level materials and designs cost less than more luxurious options. The price of an extension can rise considerably, for instance, if you use special foundation materials, add luxury elements, or introduce a lot of glazing.

Whether You’re Extending A Kitchen Or Bathroom

Creating a rear conservatory or an extra living space is generally cheaper than a new kitchen or bathroom extension. The cost of fittings and fixtures is typically lower in the former than it is in the latter.

Bathrooms tend to cost more because of all the additional plumbing and electrical work required, as well as expensive fixtures and fittings, like toilets, showers, and bath units. Typically, people need to add around £5,000 on top of the estimated cost to get a sense of the overall price of installing a new bathroom.

Likewise, kitchen extensions require lots of additional work not required for your average conservatory. Not only do you have to factor in new water and electric connections, but also cabinetry, appliances, and the obligatory kitchen island.

Trees In The Wrong Place

Trees are surprisingly costly to remove. Often, you need to hire a specialist tree removal company and stump grinder to get rid of the base and root. These professionals, however, need to be able to get their equipment to the site of the tree, which isn’t always possible.

What’s more, many trees have something called a Tree Preservation Order. A TPO is a legal document which says that the homeowner isn’t allowed to chop down a tree for development, even though they own it. Trees, therefore, could force up your extension costs substantially.

How to get more Value for Money from your Extension?

If you want to get more value for money on your extension, what can you do? Take a look at some of these ideas.

– Use on-site tradesmen to build a quality kitchen for less than the high-street brands
– Build a double-story extension to get the most value per metre-squared
– Use cost-effective blockwork
– Choose long-lasting materials


So what have we learnt?

The main takeaway is that the cost of an extension is between £2,500 and £3,000 per square metre.

You’ll pay more for higher quality materials and when extending rooms which expensive fittings, like kitchens and bathrooms.

Planning constraints may also increase costs, especially if you want an extension which falls outside of current “permitted development” rules.

Finally, your current situation may increase costs, such as building on an inaccessible site or one covered by protected trees.

How To Get Planning Permission In London - Ebook

This guide is suitable for anyone requiring consent from the local council to alter a home. It reviews the ins and outs of UK planning and strategies for successfully navigating it, based on our own experience.

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