How much does a two storey extension cost?
Better value per square metre than single storey, because you only pay for one roof and one set of foundations.
These are Extnd estimates built from recent UK project costs, not official statistics. Treat them as a planning range rather than a quote.
Where the money actually goes
Based on a £110,000 project. The proportions hold reasonably well across the range, which is useful when you are sanity checking a quote.
| Line item | Typical cost | Share |
|---|---|---|
| Design, drawings and planning | £11,000 | 10% |
| Structural engineering | £4,400 | 4% |
| Groundwork and foundations | £17,600 | 16% |
| Structure, walls and roof | £26,400 | 24% |
| Glazing and external doors | £13,200 | 12% |
| Roofing and weatherproofing | £7,700 | 7% |
| First fix, electrics and plumbing | £9,900 | 9% |
| Plaster, floors and decoration | £12,100 | 11% |
| Building control and fees | £3,300 | 3% |
| Contingency | £4,400 | 4% |
What moves this number
- Drainage. If a soil stack or manhole sits where you want to build, budget several thousand before anything else happens.
- Structural openings. Wide steels need bigger foundations at both ends. A five metre opening is not twice the price of a two and a half metre one.
- Glazing. The single biggest lever. Standard doors to slim framed sliders can be a fifteen thousand pound swing.
- Ground conditions. Clay, trees nearby or a high water table all push foundations deeper.
- Where you are. London and the South East run roughly twenty to thirty per cent above the national average on labour.
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Questions people ask before they commit
How much does a two storey rear or side extension cost in 2026?
Most projects land between £70,000 and £175,000, with the middle of the market around £110,000. The spread is wide because specification moves the number far more than size does. Two identical footprints can differ by thirty thousand pounds on glazing and finishes alone.
How long does a two storey rear or side extension take?
Allow 16 to 24 weeks on site once work begins. Before that, design and planning typically take eight to sixteen weeks depending on whether you need permission. The build is the predictable part. The approvals are where projects slip.
Do I need planning permission for a two storey rear or side extension?
Often not. This type usually does need planning permission rather than permitted development, because of its size or position. The Project Calculator will tell you which side of the line you are on in about a minute.
What makes the price go up the most?
Three things, in order. Moving drainage or a soil stack. Structural openings that need steel, particularly wide ones. And glazing, where the jump from standard doors to a slim framed sliding system can add fifteen thousand pounds on its own. Size is a surprisingly small factor by comparison.
Is it cheaper to extend or to move?
Extending almost always wins on pure cost. Stamp duty, agent fees, legal costs and removals on a typical UK move comfortably exceed the cost of a well judged extension, and you keep the house, the street and the school catchment. The exception is when the house cannot take what you need.
Compare with other projects
Single storey rear extension
£42,000 to £95,000. Around 12 to 18 weeks on site.
See the breakdown → Cost guideLoft conversion
£38,000 to £90,000. Around 8 to 12 weeks on site.
See the breakdown → Cost guideSide return infill extension
£45,000 to £98,000. Around 12 to 16 weeks on site.
See the breakdown → Cost guideWraparound side and rear extension
£65,000 to £145,000. Around 16 to 22 weeks on site.
See the breakdown → Cost guideBasement conversion or dig out
£95,000 to £320,000. Around 20 to 32 weeks on site.
See the breakdown → Cost guideIntegral garage conversion
£14,000 to £38,000. Around 4 to 8 weeks on site.
See the breakdown → Cost guideFront porch extension
£6,500 to £22,000. Around 3 to 6 weeks on site.
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