A simple house extension calculator to help you plan your budget. These figures have been updated following recent price increases in construction costs, according to a survey of architects and designers on Design for Me.
Image above by Simon and Jon architects on Design for Me. See their full profile and shortlist them for your project here.
House extension cost calculator
The above calculator is based on estimates as a starting price for a house extension. Above costs assume modest finishes, fittings, electrics and lighting.
From our recent research and survey of architects, the starting cost per square meter is £2,400 m/2 INC.VAT (or from £2,800 m/2 INC.VAT in London/South East England).
Double storey extension
Following a rule of thumb, we have allowed for 50% extra to the build cost of a single storey extension.
Kitchen cost
We have allowed £10,000 for a kitchen (incl. of VAT). This should allow for a basic, average sized kitchen with basic appliances (IKEA or similar). You can add extra to the total as you require for your specification.
Bathroom cost
We have allowed £5,000 for a WC/bathroom (incl. of VAT). Again, this is a starting cost for modest finishes and fittings.
Large format glazing (including sliding doors/ bifolds)
We have allowed £2,000 (incl VAT) per linear metre for large format glazing. Please note this is a starting cost for a good quality, but basic, system. For bespoke details, thermal upgrades, extra large panes of glass the costs will increase.
Professional fees
We have allowed 15% on the construction costs for professional fees.
Are there any complex site constraints?
Your builder will need to include additional costs in their quotation if any of the following apply:
- Your soil type and ground conditions may present more costly construction methods / techniques.
- Limited access to the site. Will it be easy for the builder to bring in tools and machinery easily?
- Existing structure. Are you removing any structural walls? If so there will likely be additional costs for steelwork.
- Are you moving any pipework, drainage, gas meters?
Managing or reducing your project costs
Make sure you get three to five quotes from recommended builders (your architect can help compile a tender list for you) to get a firm idea of the costs before starting work.
Shop around. Designers and contractors are often able to access trade discounts.
It pays to get professional advice from a good architect. They can help you realise the main ambitions for your extension, whilst helping control your costs. They will work with you to make sure you get the most out of your budget by recommending where your budget is best spent, and where costs would be unnecessary or could be minimised.
Your architect can also help you with your building contract, to prevent any nasty surprises down the line. They can also inspect the builder’s work to ensure it’s being carried out in accordance with the agreed design and specifications.
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