Thinking of adding more space to your home? A single-storey rear extension can completely transform the way you live — but the first question most homeowners ask is the same: How much does a home extension cost in the UK?
The short answer is — it depends. The long answer is what we’ll cover here.
At Buildaway, we design and build single-storey extensions across London, Kent and Essex — so we know what affects the price, where you can save, and when to spend for long-term value.
1. Average Cost of a Single-Storey Extension in London
For 2025, you can expect the following price ranges:
💡 These prices include design, materials, labour, and VAT — based on standard single-storey extensions in Greater London.
In short: If you’re wondering “how much does a single storey extension cost?” — in London, a realistic starting point is around £2,500 per square metre.
2. Factors That Affect the Cost of a House Extension
Every project is unique, so here’s what makes the biggest difference in price:
Size and Layout
The larger your extension, the lower the cost per square metre — but the higher the overall spend. Small extensions often cost more per m² because fixed costs (like design and groundwork) stay the same.
Design Complexity
A simple rear extension with a flat roof will cost less than one with bi-fold doors, skylights, or structural steelwork.
Location
Building in South East London, Bromley, Croydon, or Greenwich typically costs less than central zones like Chelsea or Islington — where access is tighter and labour costs are higher.
Ground Conditions
Poor soil, sloping gardens, or difficult drainage can all increase foundation and excavation costs.
Finishes and Materials
High-end glazing, brick matching, underfloor heating, or bespoke joinery will all raise your total.
3. What’s Included in the Cost of an Extension?
When comparing quotes, always check what’s included. A full-service contractor like Buildaway typically covers:
- Architectural design and structural drawings
- Planning permission and building regulation approval
- Foundations and brickwork
- Roofing and insulation
- Windows, doors, and glazing
- Electrical and plumbing installation
- Internal finishes (plastering, flooring, painting)
Some builders quote only for the shell build — leaving plumbing, electrics, and finishes as extras.
4. Hidden or Overlooked Costs
Even the best-planned projects have costs homeowners forget to include:
- Party Wall Agreements — if you share a wall with neighbours (£900–£2,000 per neighbour)
- Planning drawings and approvals — around £1,000–£3,000
- Kitchen or bathroom installation — £5,000–£15,000 depending on spec
- Landscaping or garden reinstatement
- Temporary accommodation (for major refurbishments)
Tip: Use a cost of extension calculator to estimate these extras — but always confirm details with your builder.
5. How to Calculate the Cost of Your Extension
To calculate the cost of an extension, multiply your desired floor area by the average cost per m² based on finish level.
Example:
25m² x £2,800/m² = £70,000 (mid-range finish)
Then add around 10–15% contingency for unforeseen work.
6. How to Save Money Without Cutting Corners
You can reduce your single storey extension cost without compromising quality by:
- Keeping the design simple — straight lines, fewer roof changes
- Re-using existing foundations or drainage where possible
- Choosing cost-effective finishes like laminate flooring or standard glazing
- Planning ahead — avoid design changes mid-build (the biggest budget killer!)
- Using one contractor for design and build — fewer mark-ups, smoother process
At Buildaway, we combine both design and construction under one roof, helping homeowners control costs and timelines from day one.
7. How Long Does a Single-Storey Extension Take?
For a typical London home:
- Design & Planning: 4–8 weeks
- Approvals & Drawings: 4–6 weeks
- Construction: 10–16 weeks
So you’re usually looking at 4–6 months from start to finish.
8. FAQs
How much does a house extension cost in the UK?
Most single-storey extensions in the UK cost between £50,000 and £100,000 depending on size and finish.
How much is it to extend a house in London?
London builds are typically 10–20% higher than national averages due to labour and material costs.
How much will a single-storey extension cost for a semi-detached house?
Expect £60,000–£90,000 for a 20–30m² build with mid-range finishes.
Can I build cheaper by managing trades myself?
You might save short-term, but delays and coordination errors often add costs — especially with London council requirements.
Why Buildaway?
Homeowners across London, Kent and Essex trust Buildaway for their extensions because we deliver:
- Transparent quotes with no hidden extras
- End-to-end project management
- Quality craftsmanship with local experience
- Clear timelines and regular updates
- Workmanship warranty on all labour
Whether you want to extend your kitchen, add a new dining area, or open up your rear space into the garden, we’ll help you build it right — first time.