Extension Costs: What Should Homeowners Budget?

Extension costs vary widely depending on size, specification, location and whether planning permission is needed. This guide helps you understand the main cost areas — not to give a quote, but to plan realistically before you commit.

Published: June 2025 · Last reviewed: 24 June 2025

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The 10-second answer

How much does an extension cost?

Typical single-storey extensions often range from roughly £2,000–£3,500+ per square metre, depending on specification — but prices vary significantly.

What affects the price most?

Size, build quality, foundations, glazing, kitchen fit-out and regional labour rates.

Do planning costs add much?

Planning application fees are relatively modest compared with build costs — but design and consultant fees add up.

Does the planning route affect cost?

Permitted development may save planning consultant time, but build costs are similar.

Which route might apply?

Lower planning-related costs may apply if:

• Your extension is permitted development — no planning application fee.
• You do not need specialist heritage or planning consultants.

This does not automatically mean permitted development applies. Every property and proposal is different and still needs to be checked.

Additional planning-related costs may apply if:

• You need a planning application and supporting drawings.
• The property is listed or in a conservation area — heritage consultants may be needed.
• You need a lawful development certificate for certainty.
• Planning permission is refused and redesign is required.

Many projects require planning permission, but the answer depends on the individual circumstances.

For whether permission is needed, see our extension planning permission overview, and the rear, side, front and garage conversion guides. For the two planning routes, see planning permission vs permitted development.

Main cost areas for an extension

Homeowners typically budget across several categories:

• Design and drawings — architect or technician fees for plans
• Planning fees — council application fee if planning permission is needed
• Structural engineer — calculations for foundations and steelwork
• Build costs — labour, materials, foundations, roof, windows, doors
• Fit-out — kitchen, bathroom, flooring, decoration
• Building regulations or building warrant — fees and inspections
• Contingency — typically 10–15% for unforeseen issues

Build cost per square metre is the largest variable. A basic single-storey extension costs less per metre than a high-specification open-plan kitchen-diner with bi-fold doors and underfloor heating.

How extension type affects cost

• Garage conversion — see garage conversion planning permission
• Rear extension — see rear extension planning permission; often the most cost-effective per square metre
• Side extension — see side extension planning permission; may involve party wall agreements
• Front extension — see front extension planning permission; often smaller but higher visibility design costs
• Two-storey extension — more expensive but lower cost per square metre of new floor area than two separate storeys built at different times

Planning and pre-build costs

Before construction, budget for:

• Planning application fee (if needed) — set nationally, typically hundreds of pounds
• Lawful development certificate fee (optional) — see lawful development certificate explained
• Design and access statement (if required)
• Surveys: topography, drainage, trees
• Party wall agreements with neighbours if applicable

Understanding the planning route early avoids expensive redesign. Check your property with Plimbu before commissioning detailed drawings.

Typical cost examples

Cost ranges are indicative guides only — not quotes. Prices vary significantly by region, specification and installer.

Modest single-storey rear kitchen extension

Roughly 15–20 m² might cost £30,000–£60,000+ all-in depending on specification and region. Planning may be permitted development — see rear extension planning permission.

Two-storey side and rear wraparound

Larger projects often exceed £100,000. Planning permission is more likely. See side extension planning permission and extension planning permission guide.

Conservation area extension

Heritage-sensitive designs and planning consultants add cost. See conservation areas explained.

How to check your property

Before relying on general guidance, check the property and proposal together.

1. Define the size and specification you need. Rough floor area and quality level.
2. Check the planning route. See extension planning permission guide and related project guides.
3. Get outline quotes from builders. At least two to three for comparison.
4. Budget planning and professional fees. See planning permission explained.
5. Include building regulations or building warrant. See building regulations vs planning permission or building warrant vs planning permission.

Plimbu helps bring these checks together so you can understand the likely planning route before going further.

Common mistakes

• Budgeting only for build cost and forgetting planning, design and regulatory fees.
• Committing to a builder before checking planning constraints.
• Assuming the cheapest quote includes the same specification.
• No contingency for ground conditions or hidden structural issues.

Frequently asked questions

How much does planning permission cost for an extension?

Council fees are set nationally — check current fees for householder applications. Design costs are additional.

Is a permitted development extension cheaper?

Build costs are similar. You may save on planning application and consultant fees.

Do extension costs differ in Scotland?

Labour and material costs vary by region. Building warrant fees apply instead of building regulations. See planning permission in Scotland.

How can I check planning before spending on design?

Check your property with Plimbu before you commission drawings.

Check your extension before you budget

Understanding the planning route early helps avoid wasted design and consultant fees.

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This guide is general information, not formal planning advice. For property-specific checks, start with your postcode in Plimbu.