What actually drives the cost of a house extension

Cost per square metre is only a starting point

Rules of thumb are useful for a first sanity check and misleading after that. Two 20m² extensions can differ enormously.

The big cost drivers

• Groundworks and foundations. Trees, drains, sloping sites and poor ground can add substantially before anything is visible.
• Structural openings. Removing a rear wall needs steel, and steel needs design, supply and installation.
• Glazing. Large sliding or bi-fold doors and rooflights are one of the fastest ways to spend money.
• Roof type. Flat is generally cheaper than pitched; a lantern adds cost and light.
• Services. Moving a kitchen or adding a bathroom pulls plumbing, drainage and electrics into the job.
• Finish level. Standard, mid or high specification changes the total more than most people plan for.

Costs that are easy to forget

1. Design, structural engineering and planning or building control fees.
2. Party wall matters where relevant.
3. Making good and decorating the existing rooms the extension connects to.
4. Kitchens, flooring and furniture, which usually sit outside the builder quote.
5. A contingency — 10-15% is sensible on any structural work.

Common mistakes

• Comparing a builder quote that excludes the kitchen with one that includes it.
• Designing to the maximum footprint, then value-engineering the quality away.
• Assuming the neighbour's price from three years ago still applies.

Next steps

Set your finish level and glazing ambitions early. Those two decisions shape the budget more than another metre of floor area ever will.