Extension Cost Guidance

How Much Does An Extension Cost?

An extension should be the best investment you ever make. It should add value to your home as well as transforming the space. To make sure that you understand the costs involved and get this right we suggest working out how much your extension should cost in 4 steps.

  1. Look at similar extension costs and set an initial budget
  2. Use our Instant Extension Cost Calculator to get a rough estimate
  3. Consider your particular requirements in more detail to refine your cost estimate
  4. Go out to tender to contractors with detailed information to receive fixed accurate build costs

Set An Initial Budget

A good first step is to work out a budget based on average costs per m2 to build an extension that is similar to yours. This is a simple and quick way to set a budget for your extension.

London Extension Costs per m²
Basic Spec Medium Spec High Spec
Single Storey Extension £1,250 £1,850 £2,500
Double Storey Extension £1,050 £1,650 £2,250
Basement Extension £2,100 £3,100 £4,200
Refurbishment £550 £1,250 £2,100

The figures shown in this table are 2019 average construction costs for London. Other parts of the country could be up to 20% less depending on the location and site condition. The average house extension cost in London in 2019 is between £1,600 – £2,100 per m². The cost of a typical high spec glass extension is around £2,500 per m². Basement extensions cost on average between £3,500 – £5,000 per m².

Extension Cost Calculator

Fill in the boxes below and in just a few clicks get an instant Extension Cost Estimate for your project in £, and £ per m2.

Refine Your Cost Estimate

Remember that your estimate of the extension cost is part of the overall costs of your project. It will usually be around 60% of the actual spend. You will need to add 20% VAT (usually), and around 10% each for statutory approvals and fees.

Detailed Extension Costs

To get beyond the ballpark figures we can calculate the actual construction cost of an extension in more detail. Although the cost of building an extension is made up of a large number of elements, you can think of it in 4 main categories. These typically make up the following proportions of the overall cost of an extension.



Some parts of the work, such as the Prelims have costs which are based on the complexity of the site and the location of the work. So you may not be able to do much about these extension costs, and they just have to be accepted as part of the work. For example on sites where access is difficult or there are unusual restrictions the cost of the Prelims could be twice as much.

Structure and Envelope

However other extension costs are more design driven, so once you understand what these are you can control them if necessary. For example there may be 2 different structural options to consider, and one may be 25% less expensive than the other. Or you may want to consider several options for the glass doors as part of the envelope that could cost either £5,000 or £15,000 depending on your choice. So these are parts of the design where you can start to refine your cost estimate as necessary.


But overwhelmingly the easiest place to have a big effect on the projected cost is in the fittings and finishes. Floor and Wall Finishes could be £30 per m2 to £300 per mdepending on what you choose. Kitchens can be £10,000 or £100,000 depending on the supplier and features. And extra finishing touches such as under floor heating, advanced lighting systems, ceiling speakers can sway the overall cost of the extension considerably.

Be Happy With Your Estimate

So once you have made design and specification choices on all of the key items you will be able to see where the cost sits within your overall budget, and adjust your choices as necessary so that it suits the amount you want to spend. Be happy with your estimate before you go out to tender as it is always best to be as certain as you can that the costs you will get back from builders will be acceptable. This approach is great as it means you are not in the dark, and it gives you control over the cost of your extension, as well as allowing you to measure the impact of each of your decisions.

To get in to even more detail with estimating costs, the Building Cost Information Service BCIS or SPONS Price Book have published schedules of rates for building projects. Using their rates it is possible to estimate the cost of an extension in great detail based on recent real world data from similar projects, and to check what the average costs of detailed parts of the work should be. The rates that they publish are very detailed, but they can be a useful tool for architects or quantity surveyors to estimate costs.

Get Accurate Build Costs

By this stage you will hopefully have a good idea of how much your house extension will be, what parts will cost the most, and where you can change your choices to control the cost if you need to. However the only way to get accurate build costs for the work is to go to tender to contractors with detailed information for them to price.

Schedule Of Works

The detailed extension cost estimate that we looked at above, can now be turned in to a Schedule of Works which can be used to tender the work. The schedule is split down in to all of the key parts of the work, and you should now already have a good idea of what each bit is likely to cost. It is sent out blank (without any cost information) to contractors for them to price.


To get the right extension cost from a builder it is important to tender with good, detailed drawings and spec. These should show the builder exactly what they need to do. Use a good architect to guide you through this and keep some key points to keep in mind to get the right price.

  • Estimates are great during the design stages. But you do not want an estimate from a contractor. You need a clear fixed price for the work, because this is the only way to have cost certainty on site.
  • Follow a proper tender process to get proper prices. Your architect can help to select and invite builders to tender for the work. Give them 4 weeks to price the work and open all the tenders at the same time. Always follow the NBS Guide To Tendering as this will make the rules of the tender clear.

If you follow these points you should be well on the way to getting the right house extension cost. Also make sure that you chose the right Building Contracts For Extensions as this can affect the extension cost too.

Good luck with your project, and if you have any questions just get in touch, we will be happy to help.

© 2PM Architects