Building Regulations for rear extensions

What does Building Regulations approval involve?

Every rear extension in England needs Building Regulations approval, whether or not planning is needed. You can use the council's building control service or a registered building control approver, by full plans (checked before work, usually within five weeks) or a building notice, and you get a completion certificate at the end.

What has to be shown on the drawings?

Foundation depth and width, the floor, wall and roof build-ups with insulation, the steel beam and its bearings, drainage runs and any build-over, fire separation near boundaries, escape routes, ventilation for the new room and the old room behind it, and the glazing specification. The structural engineer's calculations go with them.

New fabric must meet Approved Document L (2021 edition incorporating 2023 amendments): walls 0.18, roof 0.15 and floor 0.18 W/m²K, windows 1.4 W/m²K or Band B. Under the simple route, new windows, doors and rooflights should not exceed 25% of the extension floor area plus any openings removed; more glass is possible with a whole-house calculation.

What does it cost?

Construction drawings and structural calculations typically cost £2,500–£5,500 including VAT for a single-storey extension and £4,000 to £7,000 for two storeys. Building control fees are usually £650 to £1,300 for a single-storey extension and are paid to the council or the approver.

What is included?

• Construction drawings
• Structural engineer's calculations
• Energy and glazing checks
• Drainage and build-over checks
• Building control application
• Site inspections coordinated
• Completion certificate
• Electrical and gas certificates handed over