In 2026, the appetite for home extensions in the UK remains stronger than ever. With property prices and stamp duty making “moving up” a costly endeavour, “improving rather than moving” has become the strategic choice for British homeowners. However, the landscape has changed: material costs have stabilised at a higher baseline, and the Future Homes Standard now demands stricter energy efficiency.
This guide provides a comprehensive breakdown of house extension costs and essential planning advice to help you navigate your project with confidence.
1. Estimating House Extension Costs (2026 Benchmarks)
Budgeting for an extension is no longer a “finger in the air” exercise. In 2026, you should categorise your costs based on the specification level and regional location.
Average Cost per Square Metre ($m^2$)
For a standard single-storey extension, expect the following price brackets (excluding VAT at 20%):
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Basic Finish: £1,800 – £2,200 per $m^2$
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Mid-Range Finish: £2,200 – £3,000 per $m^2$
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High-End/Premium Finish: £3,000 – £4,500+ per $m^2$
Typical Project Costs
| Extension Type | Typical Size | Estimated Cost (Inc. VAT & Fees) |
| Small Rear Extension | 20$m^2$ | £48,000 – £65,000 |
| Medium Kitchen/Diner | 30$m^2$ | £72,000 – £95,000 |
| Large Wrap-around | 50$m^2$ | £120,000 – £160,000 |
| Two-Storey Extension | 60$m^2$ (Total) | £130,000 – £180,000 |
Note on Two-Storey Builds: Interestingly, a two-storey extension is often more cost-effective per square metre than a single-storey one, as the expensive elements—foundations and the roof—are shared across two floors.
2. Planning Advice: Do You Need Permission?
One of the biggest hurdles is understanding the “Planning Maze.” In 2026, many extensions fall under Permitted Development (PD) rights, meaning you don’t need a full planning application.
Permitted Development (PD) Rules for 2026
You can typically build without full planning permission if:
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Depth: For a detached house, you can extend by up to 4m (rear). For semi-detached or terraced, it is 3m.
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The Larger Home Extension Scheme: Under “Prior Approval,” these limits increase to 8m (detached) and 6m (others), provided neighbours are consulted and do not object.
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Height: The extension is no higher than the highest part of the existing roof. Single-storey eaves height is max 3m if within 2m of a boundary.
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Footprint: The extension and other buildings do not take up more than 50% of the total land around the “original” house.
When You Must Apply for Planning Permission:
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You live in a Conservation Area, Area of Outstanding Natural Beauty (AONB), or a National Park.
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Your home is a Listed Building.
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You are building a Wraparound Extension (these rarely fall under PD).
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You want to use materials that differ significantly from the original house.
3. The “Hidden” Costs: Professional Fees & Extras
The “build cost” is only part of the story. You must factor in professional fees, which typically add 10% – 15% to your total budget.
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Architect Fees: £2,500 – £6,000 (depending on whether they manage the project).
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Structural Engineer: £1,000 – £2,500 (essential for calculating RSJ beam sizes).
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Planning Application Fee: Approx. £258.
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Building Regulations Inspections: £500 – £1,000.
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Party Wall Agreement: £1,000 – £2,000 per neighbour (if you are building on or near a boundary).
4. 2026 Trend: The “Future-Fit” Extension
Building in 2026 means adhering to the latest Building Regulations (Part L), which focus on thermal performance.
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The “Green” Splurge: Investing in triple glazing, Air Source Heat Pumps (ASHP), and underfloor heating may increase your initial outlay by 10%, but it significantly boosts the resale value and reduces long-term running costs.
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Biophilic Design: Using oversized sliding doors and skylights is now a standard requirement for buyers looking for “indoor-outdoor” living.
