How much does a house extension cost to build?
Indicative UK construction costs for early budgeting, with a simple calculator and advice on the additional fees to consider before starting a project.
Planning an extension to your home in Harlow, Epping, Chelmsford or elsewhere in Essex? It helps to understand the likely construction budget before investing in detailed plans. The figures below are national guide prices, not quotations from Essex builders. Actual costs depend on access, ground conditions, size, design and specification.
Single-storey extension cost calculator
Based on £1,800–£3,000 per m² for a single-storey extension. These figures exclude VAT and separate project costs. If the slider does not work in your browser, use the examples below.
Example construction budgets
| New floor area | At £1,800/m² | At £3,000/m² |
|---|---|---|
| 15 m² | £27,000 | £45,000 |
| 20 m² | £36,000 | £60,000 |
| 25 m² | £45,000 | £75,000 |
| 30 m² | £54,000 | £90,000 |
| 40 m² | £72,000 | £120,000 |
What additional costs should you allow for?
Construction cost-per-square-metre guides do not necessarily include everything needed to complete your project. Ask builders to confirm their assumptions and whether the quotation includes VAT, excavation and drainage, structural alterations, kitchen fittings, specialist glazing, heating and electrical work, internal alterations to the existing house, waste removal and final finishes.
Allow separately for planning and building regulations drawings, planning application charges, building control, structural engineering where required, other specialist reports and an appropriate contingency. The FMB suggests considering a contingency of around 10–15% for unforeseen work.
What about two-storey extensions?
Two-storey extensions can cost more overall while offering a lower cost per square metre of added floor area because the foundations and roof serve both storeys. Published cost guides vary significantly in their assumptions, so the single-storey calculator above should not be used as a two-storey quotation. Obtain a project-specific builder’s estimate.
How do planning drawings fit into your budget?
Robert Blake Designs prepares residential planning drawings and assists with planning applications, including house extensions, loft conversions and garden annexes. Stage 1 typically includes an initial site visit, discussion of the proposals, a measured survey and the agreed planning drawings. Where appropriate, Stage 2 building regulations drawings are quoted separately. Structural engineering, structural calculations and building work are not provided by Robert Blake Designs.
For our drawing fees, visit the RBD price list. You can also read about house extension planning drawings, building regulations drawings and our projects.
Planning an extension in Harlow or Essex?
Discuss your ideas with Robert Blake Designs and obtain a quotation for the planning drawings appropriate to your project.
Frequently asked questions
Are these local builder quotations?
No. The ranges are indicative UK-wide published figures. Local Essex quotations may differ, particularly where access, groundwork or specifications are challenging.
Does the calculator include VAT?
No. VAT, where applicable, should be allowed for separately. Confirm the VAT treatment with the contractor.
How often are the cost figures checked?
We aim to review the published source guides regularly and revise this page when material changes are verified. The check date above identifies when these figures were last reviewed; the estimates are not live quotations.
Disclaimer: This is general budgeting information only. RBD does not provide building quotations or cost consultancy. Obtain itemised quotations from suitably qualified builders before committing to construction expenditure.
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