How much does a house extension cost?

Cookridge extension - rear view after

One of the most common questions asked and of course one of the most important factors as to the viability of any construction project. How much will it cost?

It is however never a simple answer with several factors influencing the final construction project costs.

A rough example based on £ per m²

If we look at this simply to start with you can work it out with a rough budget cost on £ per m². A few years ago this was pretty simple maths at £1000 per m². General build costs have risen and it would probably be more advisable to allow £1200 per m². So, for example, if you are building a 3m by 4m single storey extension this would come out at £14,400 (3 x 4 x 1200).

If you are planning a 2 storey extension the upper floor would be an additional half of the £ per m². This is because much of the building work required for 1 storey is only needed once, regardless of storey number, such as the foundations and roof. So if the same extension was 2 storey it would come out at £21,600 (3 x 4 x 1200 x 1.5).

This figure covers the general build including basic plumbing and electrics to a reasonable standard but not expensive finishes such as tiling. If you are looking for a high end finish this will increase the costs per m², and if your extension includes a kitchen or bathroom these will be extra too.

Other influences on the cost of an architectural project

Other factors to take into account which are hard to put a figure on are those that are site and/or project specific. For example;

  • The type of construction method and materials you use can have a big impact. Using non-traditional construction techniques could increase the costs, as could using stone instead of bricks.
  • The site itself could be difficult to build on. If the extension requires additional ground works such as deep foundations this will add more to the construction costs.
  • Unexpected costs. For example, moving a gas meter might seem like a minor thing but can easily cost £500 – £1000. Connecting into your existing drainage may not be straight-forward and require more work than expected.
  • Off-site project costs. Fees for architects, engineers, planning and building regulations etc. These can add on a possible 10% – 20% on top of the build costs.

What contingency is recommended for my construction project?

A good sized contingency is recommended on any project; 10% is normally advisable. Hopefully it won’t be needed but it is best to be prepared as, working with an existing building in particular, can throw up some unexpected issues that will need to be resolved.

If you are thinking of an extension, or any building work, it is best to speak with an Architect who can advise you on the specifics with your project. A good Architect will also provide you with alternative designs that can reduce your build costs while still getting the result you require. Most builders will give a free quote for the works, but the more information and detailed drawings you can provide them with, the more accurate their cost estimates will be.


  • Jimbo James

    Thanks for the article. The post doesn’t have a date, are these still roughly valid in 2014?

    • James Butterworth

      Hi, Glad you like the article, it was written earlier in 2014 so the costs should still be valid. However construction costs are tending to increase quicker than inflation at the moment.

  • Eric Henderson

    You don’t say if these costs include VAT. Most builders seem to quote Ex vat prices so should we add 20% to these figures or is that it? A local builder quoted me 1500/sq m Ex VAT!

    • James Butterworth

      These costs would not include VAT. It depends on the builder some small builders are not limited companies and so would not charge VAT. But always check as there maybe some VAT to pay on materials that the builder passes on from his suppliers. If in doubt always ask.
      £1500 per m² is quite high. If you are going for a very high end finish it could get to this cost, but it does seem excessive. However, regional differences can increase costs (London being more expensive for example) and the availability of local builders can push up prices with supply and demand.

  • Michael cusick

    Every biuldig is different and prices vary be under under no illusion what your neighbours paid is goin to be the same .foundations may be deeper detail on drawing totally different could make a vast difference. An 14000 for a singel extention is more like 17000 in real money

    • James Butterworth

      Yes, every build is different due to specifics of the site & client requirements. This article is just intended as a guide. To get an accurate price you would need to get detailed drawings and specifications completed and then priced by a builder.

  • Scott


    Came across your article from a google search.

    As a builder its very refreshing to finally see a move away from the dreaded £1k per sqM figure that seems to have been around for ever. You’d be amazed at the amount of potential customers who’ve heard from architects/friends/family and who believe that figure is relevant today and wonder why the quotes for their build aren’t near that.
    I remember £1kSQm from circa 2004 and the increase in insulation alone from then is incredible, not just the cost of the stuff but the size requirements. For example 10 years ago 95×45 timbers were used for a timber kit with 90mm kingpin now its 145×45 with 140mm kingpin minimum.

    I price every job individually so try to stay away from a set pricing structure however I would say that £1200 + VAT is a good guideline.

    All the best,


  • MrThin

    Perhaps a naive question, but are these costs assuming a specific roof type for the extension?

    • admin_studioj

      This would assume a standard traditional pitched roof

    • James Butterworth

      This would assume a standard traditional pitched roof