In 2026, the mantra for UK homeowners has shifted from “move on” to “build on.” With property prices remains high and the logistical nightmare of moving—incorporating Stamp Duty, legal fees, and estate agent commissions—adding square footage to your existing home is the most logical way to gain space while increasing your property’s equity.
However, the construction landscape has changed. With the Future Homes Standard and updated Building Regulations (Part L) now in full effect, budgeting for an extension requires more precision than ever. This guide breaks down the realistic house extension costs in the UK for 2026.
1. Average Extension Costs Per Square Metre ($m^2$)
Most UK builders and architects provide initial estimates based on the square metre. In 2026, these figures are heavily influenced by the “quality of finish” and regional labour rates.
| Build Quality | Typical Cost per m2 (Ex. VAT) | What’s Included? |
| Basic Finish | £1,800 – £2,200 | Standard brickwork, uPVC windows, radiator heating. |
| Standard Finish | £2,200 – £2,800 | Better glazing, mid-range flooring, integrated lighting. |
| Premium Finish | £2,800 – £3,500+ | Large format glazing, underfloor heating, bespoke joinery. |
Note: If you are building in London or the South East, expect to add 20%–30% to the figures above due to higher labour demands and logistical complexities.
2. Costs by Extension Type (2026 Averages)
The type of extension you choose significantly dictates the structural requirements and, consequently, the price.
Single-Storey Rear Extension
The classic kitchen-diner expansion.
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Small (20$m^2$): £40,000 – £55,000
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Medium (30$m^2$): £60,000 – £85,000
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Large (50$m^2$): £100,000 – £145,000
Double-Storey Extension
While the total price is higher, double-storey extensions offer better value per square metre because they share the same foundations and roof as a single-storey build.
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Standard 60$m^2$ (30$m^2$ per floor): £110,000 – £160,000
Side Return Extension
Common in Victorian terraces, these utilise the narrow alleyway at the side of the house.
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Typical Project: £35,000 – £65,000
Wrap-Around Extension
Combining a side-return and a rear extension, these create a massive L-shaped living space but involve significant structural steelwork.
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Typical Project: £85,000 – £140,000
3. The “Hidden” Costs: Professional Fees & VAT
When people ask “how much does an extension cost?”, they often forget the 20% “hidden” tax and the experts required to get the project off the ground.
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VAT (20%): Unless you are building a new house from scratch, most extension work is subject to 20% VAT. Always check if your builder’s quote includes this.
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Architect Fees: Usually 7% – 15% of the build cost. For a 30$m^2$ extension, budget £2,500 – £5,000.
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Structural Engineer: Essential for calculating steel beam sizes. Budget £1,000 – £2,500.
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Planning & Building Regs: * Planning Application (England): £258
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Building Regulations Inspections: £500 – £1,000
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Party Wall Agreements: If you are building on or near a boundary, you may need a surveyor. Budget £1,000 – £2,000 per neighbour.
4. Fitting Out: Kitchens and Bathrooms
An extension is just a shell until you fill it. The “wet” rooms are the most expensive part of any build.
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New Kitchen: £15,000 – £35,000 (Standard to Mid-range)
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New Bathroom/En-suite: £5,000 – £10,000
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Glazing (Bi-fold/Sliding Doors): £1,500 – £2,500 per linear metre.
5. 2026 Factors: The “Eco-Uplift”
The Future Homes Standard means that extensions in 2026 must be more energy-efficient.
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Insulation: Expect to pay slightly more for thicker, high-performance insulation to meet new U-value targets.
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Heating: Many homeowners are now opting for Air Source Heat Pumps (£7,000 – £13,000) or Electric Underfloor Heating (£60–£100 per $m^2$) as gas boilers are phased out in new specifications.
