House Extension Cost Guide
How much does a house extension cost in Hertfordshire and London? We break down real prices by extension type, what affects the cost, and how to get the best value from your build budget.
Extension Costs by Type
These are typical costs for house extensions in Hertfordshire based on our recent projects. London prices are generally 15–25% higher.
Single-Storey Rear (3m × 5m)
The most popular extension type. Often falls under Permitted Development, meaning no planning permission required.
Single-Storey Rear (5m × 8m)
A larger open-plan kitchen-diner or family room. Better value per square metre than smaller extensions.
Double-Storey Extension
Typically a kitchen/living room below and bedroom/bathroom above. Excellent value — roughly 50% more space for 30–40% more cost.
Side Return Extension
Perfect for Victorian/Edwardian terraces. Reclaims the wasted alleyway to widen your kitchen or living room.
Wrap-Around Extension
Combines a rear and side extension for maximum new space. Ideal for creating a large kitchen-diner.
Prices are estimates based on recent Leeway Brothers projects in Hertfordshire (2025–2026). Final costs depend on individual project requirements, ground conditions, specification, and access. Request a free quote for an accurate price tailored to your property.
What Affects the Cost?
Understanding these six factors will help you budget accurately and make informed decisions about your extension project.
Size & Footprint
The biggest cost driver. Larger extensions cost more overall but less per square metre. A 40m² extension typically costs 30–40% less per m² than a 15m² extension.
Number of Storeys
A double-storey extension shares foundations and roof costs across two floors, making the upper floor significantly cheaper per m² than building single-storey.
Specification & Finishes
Standard finishes (painted plaster, vinyl flooring, basic kitchen) vs. premium finishes (underfloor heating, bi-fold doors, stone worktops) can add 30–50% to the fit-out cost.
Ground Conditions
Clay soils, trees nearby, or high water tables can require deeper or engineered foundations, adding £3,000–£10,000 to groundwork costs.
Access & Location
Difficult access (narrow alleyways, no rear access) increases costs. London prices are typically 15–25% higher than Hertfordshire due to labour and logistics.
Planning & Structural Work
If your extension needs planning permission, party wall agreements, or significant structural steelwork, budget an extra £3,000–£8,000 for professional fees.
Extension vs. Moving Home
For many Hertfordshire and London homeowners, extending is significantly cheaper than moving — and you keep the home you love.
Extend Your Home
- Single-storey extension: £30,000–£80,000
- Double-storey extension: £60,000–£120,000
- No stamp duty or estate agent fees
- No chain, no risk of sale falling through
- Keep your school catchment area
- No disruption of moving for the family
- Adds 10–20% to your property value
Move House
- –Stamp duty: £5,000–£25,000+ (depending on price)
- –Estate agent fees: £5,000–£15,000
- –Solicitor/conveyancing: £1,500–£3,000
- –Removal costs: £1,000–£3,000
- –Surveys & searches: £500–£1,500
- –Potential renovation of new property
- –Total moving costs: £15,000–£50,000+
Common Questions About Extension Costs
How much does a house extension cost in 2026?
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In 2026, a single-storey rear extension in Hertfordshire typically costs £30,000–£80,000 depending on size and specification. Double-storey extensions range from £60,000–£120,000. London prices are typically 15–25% higher. Contact us for a free, detailed quote for your specific property.
How much does a house extension cost per square metre?
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Extension costs per square metre in Hertfordshire typically range from £1,500–£3,300/m² depending on the type of extension, specification level, and your property. Single-storey extensions tend to cost more per m² than double-storey because the foundations and roof cost is spread over less floor area.
Is a house extension cheaper than moving?
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In most cases, yes. Moving costs (stamp duty, estate agent fees, solicitor fees, removal costs) typically total £30,000–£60,000+ in Hertfordshire and London. An extension adds the space you need while avoiding these costs and the disruption of moving. A well-built extension also adds value to your property.
Do I need planning permission for a house extension?
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Many single-storey rear extensions fall under Permitted Development rights and don't need planning permission, provided they meet size and distance limits. Larger extensions, front extensions, and those in conservation areas usually need planning approval. We assess this at your free consultation and handle the entire process if permission is required.
How long does a house extension take to build?
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A single-storey extension typically takes 10–16 weeks from breaking ground. A double-storey extension takes 14–20 weeks. Add 8–12 weeks for design, planning permission (if needed), and building regulations approval before construction begins. We provide a detailed schedule with every quote.
What is included in a house extension quote?
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Our extension quotes include everything from groundwork and foundations to the finished, decorated room — structural work, roofing, windows and doors, plumbing, electrics, plastering, flooring, painting, and building control sign-off. We don't leave hidden extras. Architect and structural engineer fees are quoted separately if required.
Get an Accurate Quote for Your Extension
Every property is different. The best way to know exactly what your extension will cost is to get a tailored quote from our team.
Ready to Start Your Project?
Get a free, no-obligation quote from our experienced team. We'll discuss your vision and provide expert advice.