Extension costs vary significantly based on size, specification, and location. As a rough guide, single-storey extensions typically cost £1,500 to £2,500 per square metre. Two-storey extensions cost £1,800 to £3,000 per square metre. A typical 20 square metre single-storey rear extension costs between £30,000 and £50,000. London and the South East are 20% to 40% more expensive than other regions.

The main factors are size, number of storeys, build complexity, specification level, and location. Structural changes like removing load-bearing walls add cost. High-end kitchens or bathrooms within the extension increase the price significantly. Difficult access, sloping sites, and poor ground conditions also push costs up. Your choice of materials, windows, and finishes has the biggest impact on final price.

A basic single-storey rear extension costs £1,500 to £2,000 per square metre. Mid-range specifications with bi-fold doors and a good kitchen run £2,000 to £2,500 per square metre. High-end finishes push this to £2,500 to £3,500 per square metre. For a typical 4m x 5m (20 sqm) extension, expect to pay £30,000 to £70,000 depending on specification.

Two-storey extensions cost £1,800 to £3,000 per square metre, but you get double the floor space for less than double the price since you share foundations and roof costs. A typical 3m x 6m two-storey extension (36 sqm total) costs £65,000 to £110,000. They offer better value per square metre than single-storey if you need the space.

Common hidden costs include planning permission (£258), building regulations (£500 to £1,000), architectural drawings (£1,500 to £5,000), structural engineer fees (£500 to £1,500), Party Wall Act surveyors (£700 to £1,500 per neighbour), building control inspections, utility diversions, skip hire, and temporary accommodation if needed. Budget an extra 10% to 15% for contingencies.

Architects typically charge 7% to 15% of the build cost, or a fixed fee of £3,000 to £10,000 for a standard extension. Architectural technologists and designers charge less, usually £1,500 to £5,000. The fee covers initial designs, planning drawings, building regulations drawings, and may include project management. Get quotes based on your specific project scope.

Moving house typically costs 8% to 10% of the property value in fees, stamp duty, and expenses. For a £400,000 house, that's £32,000 to £40,000 before buying somewhere bigger. An extension often costs similar money but adds value to your current home without the disruption of moving. Extensions make most financial sense when you like your location and the cost is less than the value added.

Keep the design simple with a straightforward rectangular footprint. Choose a flat roof instead of pitched, which costs less to build. Stick to standard window sizes. Avoid moving drainage or gas mains if possible. Get three to five quotes and compare like for like. Consider a main contractor versus managing trades yourself. Phase the work if budget is tight, completing the shell first.

A well-designed extension typically adds 50% to 75% of its cost to your property value. A £50,000 extension might add £25,000 to £37,500 in value. Adding a bedroom or bathroom generally adds more value than extending a living space. Quality of finish matters significantly. Over-extending for your street can limit returns, so check comparable sold prices before committing.

Cost per square metre typically includes foundations, walls, roof, windows, doors, basic electrics, plumbing, plastering, and decoration. It usually excludes kitchen units, bathroom fittings, flooring, planning fees, professional fees, and landscaping. Always clarify what's included when comparing quotes. A cheap headline rate often excludes items that push the final cost much higher.