How Much Does a House Extension Cost?

How Much Does a House Extension Cost?

How Much Does a House Extension Cost?

The most frequently asked question is, how much would an extension cost. We thought we should make a blog post on it. How much an extension costs can vary significantly, from the ground level to the type of materials required. It can be vary hard to price an extension over the phone without physically seeing any plans or the site itself. Builders have to take into consideration a lot of factors i.e can they get the scaffolding in to build the extension. In this blog we will give you a rough idea how a builder may price an extension

Single Storey Extension

There is an outdated rule of thumb where by it was £1,000 per m2, however the figures are now averaging £1300 per m2 to £1500 per m2. This rule will give you an extension, plasterboard with white switches, sockets and a coat of paint.

Let take an example, you plan to build an extension 6metres in depth and 5m in width

6 x 5=30m2

30m2 x £1,300 = £39,00

There will be other professional fees on top which, you should allow an extra 8% for professional fees i.e structural engineer.

There will be VAT to be added to the value.

Single storey cost would be in the region of £50,540 at a rough cost.

There is a cheaper alternative wherby you instruct us for the complete design and build of your extension or loft conversion. We will give you a fixed price for the whole project and it will be left to us to manage the whole project including any fees, which needs to be paid to the council. There are a lot of advantages with this option.

  • Design and build by us will be much cheaper than instructing different professionals.
  • We have our in house designers and structural engineers.
  • This option will save you a lot time as one company will be managing the whole project. We will work hand in hand with our designers and structural engineers to make sure you project is delivered to your requirements
  • The Vat element may be cheaper aswell. You will be paying a flat fee and not paying VAT to different companies.

 I want to build a two-storey extension

A general rule of thumb is to allow anywhere between 50% and 55%. Two-storey extension tends to be smaller than single storey extensions.

In the above scenario the cost was £39,000 excluding VAT and Professional fees

£39,000 +19,500= £58,500

8% Professional fees = 4,680

VAT at 20% = 11,700

Total price for a two storey extension = £70,200

This price would be for a two storey not including wrap around two storey or a two with a single rear extension. This again would be for the structure, roof, heating, plastered walls, one coat paint, windows, Electrics and tongue and groove flooring.

I want to add a bathroom and fitted kitchen would the price include this

The price will include plumbing supply for a bathroom however it will be up to a customer supply. The main reason a builder will not include a price on a complete fit of a bathroom is because everyone taste is different.

We would allow for a minimum of £3500 to include tiles, grout, sanitary ware and finishing’s i.e radiators, basin taps and so fourth.

The price will not include a fitted kitchen. Everyone has a different vision of how they would like their kitchen to look. We would allow an extra £15,000 to provide for the whole kitchen. £15,000 would get you a mid range kitchen from a supplier and fitted by us.

Another point to add to the price would be fittings. In the cost mentioned above this would be for white furniture fittings, you may want to upgrade to chrome fittings. You may choose to tile your extension or lay carpet of engineered flooring. This can have a substantial impact on how much it would cost, every type of floor has a different price, which is something you need to bear in mind.

Aluminum bi folding doors or sliding doors are the latest trend this will also be an added expense to your beautiful extension in London.

Are there any other factors, which may add more cost

Soil type is very important; this may impact how deep your foundation is to be determined by your council if your planning to use Building notice. If you have applied for full building control application then we will have all the information to hand.

Another big factor is structural work you would like for example opening the downstairs completely this would be additional cost and will not be able to price it until we physically see the property.

Drainage system – if you’re planning to move existing pipe work or drainage this can may be additional charges. This could also mean obtaining a Build Over Agreement from Thames Water.

Gas metre and fusebox – if you’re planning to move any of these two it will be additional charges.

Painting and decorating anything other than the extension this could be extra charges.

How can I reduce my building cost?

It is imperative to get the right builder who has experience within the trade, of course with experience comes the price. It better to pay the industry standard and get the job completed efficiently without any hassle than to save a few pounds and employ someone much cheaper. You should always see your builder’s previous work to see the finishing of their work. ASK QUESTIONS? We cannot stress how important this is. Ask your builder questions regarding your projects opinions and things you’re worried about. By employing the right builder you will save cost because you will not need to re employ another builder to fix the mistakes of your previous builder.

  1. Have a vision in mind of what you would like your property to look like from the offset and have a meeting with your builder before he starts. If you tell your builder half way through a project I would like my house to look like this after researching, your builder may have charge extra for the additional work he would need to carry out structurally. You can never be under the expression that you told your builder ALWAYS explain before the work is carried out and enter into a legally binding contract.


  1. Create a budget forecast and aim to stick to it. Have a look and furniture prices before hand to get a feel of how much it would cost you. For example if you want tiles have a look at the price for that specific tile per sqm so you understand the cost implications.
  1. A design and build company is always the best option. They will design your build before working on it and you have approved the design it should limit the additional cost.