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House extension costs: what to put in your budget
The useful starting question is not just “what does an extension cost per square metre?” It is “what must this price include to deliver the space I actually want?”
Define the finished space before comparing prices
A weatherproof shell and a finished kitchen extension are different scopes. Write down whether your project needs a fitted kitchen, heating, lighting, flooring, decorating and work to the existing house. Ask anyone providing an estimate to state where their price stops.
Area matters, but it does not describe the whole job. A smaller extension can still need site setup, drainage alterations and a major opening into the existing house. A single cost-per-square-metre figure cannot describe those differences.
Extension budget checklist
| Budget section | What to confirm |
|---|---|
| Design and investigations | Drawings, surveys, structural input and any work needed to resolve unknown site conditions. |
| Permissions and approvals | Which applications, professional services and inspection charges need separate allowances. |
| Site setup and removal | Access, protection of the existing home, demolition, skips and disposal. |
| Groundworks and drainage | What has been assumed about foundations, ground conditions, existing drains and service diversions. |
| Structure and weatherproofing | Walls, structural openings, roof, insulation, windows, doors and making connections to the existing house. |
| Services | Heating, plumbing, electrics, ventilation and alterations to existing systems. |
| Fixtures and finishes | Kitchen or bathroom fittings, flooring, joinery, lighting and decoration. |
| External work and completion | Steps, drainage outside doors, paving, garden reinstatement and clearing the site. |
| Owner’s costs | Items you buy directly, temporary accommodation if needed, professional fees and applicable tax. |
Four questions that can change the scope
- How does the new space connect? Record openings and alterations you want. A structural professional should establish the design needed; an app cannot confirm a beam or foundation.
- How will materials reach the site? Tell builders about restricted side access, parking and the space available for storage.
- How much glass do you want? Describe door and window sizes and intended specification early, rather than leaving a vague “doors included” line.
- What happens to the existing rooms? Include the old kitchen removal, changed layouts and making good where relevant.
Keep approvals separate from design assumptions
Planning permission and building regulations approval are different matters. GOV.UK’s building regulations guide explains the distinction and links to the different arrangements in Scotland and Northern Ireland. Confirm the route for your property with the relevant authority or adviser before proceeding.
Build the budget without counting items twice
Start with the agreed construction scope. Add separately priced items that are excluded, professional costs not already included, applicable tax and a contingency for uncertainty. Keep optional upgrades on a separate list.
Do not add a kitchen, design fee or site allowance again if it is already within a total. Mark every budget line as “included”, “separate” or “not confirmed”. Our renovation budget planner gives you a worksheet for doing this.
Where Estiquote fits
Use the iPhone estimator to explore a planning range for your project type, area, country, finish and relevant extras. The output is a model, not a site survey or a current contractor quotation. Project Pass adds the detailed schedule and project PDF for the active project.
This guide deliberately does not present a national average as a price for your home. Obtain site-specific written quotes, then use our quote comparison worksheet to reconcile their scope.