The honest answer to “how much will my extension cost?” is: it depends — but that’s not very helpful when you’re trying to plan. Here’s what actually drives the number, so you can budget realistically.
The main cost drivers
- Size and storeys. A single-storey rear extension costs less per square metre than a double-storey, but double-storey often adds the most value because the roof and foundations are shared.
- Structure. Knocking through load-bearing walls means steel beams and structural calculations.
- Ground conditions. Trees, drains and sloping plots can affect foundations.
- Finish. Kitchens, bi-fold doors, underfloor heating and high-end tiling all move the figure.
- Planning. Most extensions fall under permitted development, but conservation areas and listed buildings in Oxford often need full planning permission.
How to budget sensibly
Set aside a contingency of around 10–15% for the unexpected, and get an itemised quote rather than a single headline number — that’s the only way to compare builders fairly and avoid surprises later.
The best next step is a free site visit. We’ll look at what you’re planning, talk through options, and send a clear, itemised quote with no obligation.