If you are planning an extension in Aylesbury, the first number you will hear is a cost per square metre. That figure is a useful starting point, but it only means something once you know what it includes. This guide sets out realistic 2026 ranges for the Aylesbury area and explains what moves your price up or down.
For a well built extension finished to a normal standard, most Aylesbury homeowners should budget roughly £2,200 to £3,000 per square metre for a single storey extension. That usually covers foundations, structure, roof, windows and doors, insulation, plastering, electrics, heating and a basic decorated finish. It does not usually cover a new kitchen or bathroom fit out.
A two storey extension often costs less per square metre, roughly £1,900 to £2,600, because the foundations and roof are shared across two floors. The total bill is higher, but you get more space for each pound spent on groundwork and roofing.
Take a common Aylesbury project: a 4m by 5m single storey rear extension on a 1970s or 1980s semi, creating a 20m² kitchen diner. At £2,400 per m², the build comes to around £48,000. Add a mid range kitchen at £15,000 and allow for professional fees, and a realistic all in budget sits somewhere between £68,000 and £75,000.
Always check whether a quote includes VAT. A firm that is VAT registered will add 20%, which makes a large difference on a project this size.
Ground conditions matter a great deal across the Vale of Aylesbury. Much of the area sits on clay, which shrinks and swells with the seasons, so building control often requires deeper trench fill foundations, especially near trees. If roots or soft ground are found, foundations can go well beyond the standard 1 metre, and the extra concrete and digging out adds cost.
Drainage is another common surprise. Many estate homes around Aylesbury have a shared sewer running across the back garden. If your extension sits over or within 3 metres of a public sewer, you will need a build over agreement with Thames Water, and sometimes the drain has to be diverted or protected.
The build cost is not the only cost. Most projects need drawings, structural calculations and building regulations approval, and many need planning permission from Buckinghamshire Council. Plenty of single storey rear extensions fall under permitted development, but it is worth confirming this before you commit.
If you share a wall with a neighbour or are digging near their foundations, the Party Wall Act may apply. A surveyor can cost several hundred pounds per neighbour, and more if each side appoints their own.
Compare quotes on the same drawings and specification, otherwise you are comparing different jobs. Ask each builder to itemise groundwork, structure, roofing, glazing and finishes, and to state clearly what is excluded. Be wary of a price far below the others; it often means something is missing and will appear later as an extra.
A good builder will visit, look at your access, drains and trees, and give a written quote with a realistic programme. On most single storey extensions in the Aylesbury area, expect around 10 to 16 weeks on site once work begins.
It is a good starting point, but it rarely includes kitchens, fees, VAT or contingency. Use it to judge whether a project is feasible, then get itemised quotes based on proper drawings.
Not always. Many single storey rear extensions fall under permitted development, but limits apply and conservation areas have tighter rules, so check with Buckinghamshire Council or apply for a lawful development certificate for peace of mind.
Much of the area sits on clay that moves with moisture levels, and trees make this worse. Building control may require deeper foundations, which means more digging, more concrete and a higher cost.
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