Every extension and loft conversion needs Building Regulations approval, even when planning permission is not required. The regulations cover structure, fire safety, insulation (Part L), ventilation, drainage, electrics (Part P) and glazing. Under the 2022 Part L uplift, new extension walls must achieve a U-value of 0.18 W/m²K, roofs 0.15 and windows 1.4. A completion certificate is issued when the work passes final inspection.
Building Regulations vs planning permission
Planning permission is about whether you may build; Building Regulations are about how you build. The regulations apply to every extension, loft conversion, structural alteration, new bathroom, rewire and boiler, regardless of size. They are enforced by the council's building control department or an approved private inspector, who checks the work at key stages and issues a completion certificate at the end.
Part L: insulation and energy
Part L sets the thermal performance of the new fabric. Since the June 2022 uplift, new extension walls must achieve a U-value of 0.18 W/m²K, which in practice means a cavity wall with 100–150 mm of full-fill insulation or a timber frame with PIR. New roofs need 0.15 W/m²K (around 150 mm of PIR over or between rafters) and floors 0.18 W/m²K (100–150 mm of PIR under the screed). Windows and glazed doors must achieve 1.4 W/m²K, which rules out most standard double glazing and pushes projects towards high-performance double or triple glazing.
Glazing area is capped at 25% of the extension floor area plus the area of any windows removed, unless you prove compliance with a SAP calculation. Large glazed rear walls in London extensions almost always need this calculation, which ICV Buildings arranges.
Structure, fire and escape
Part A requires structural calculations for every steel beam, lintel and foundation, signed off by a structural engineer. Foundations in London clay are typically 1 m deep, or deeper near trees. Part B covers fire: loft conversions need a protected stair enclosure with FD30 fire doors, interlinked mains-powered smoke alarms on every floor and, for three-storey houses, either an escape window or a sprinkler alternative. Extensions must not block the escape route from upstairs windows.
Drainage, ventilation and electrics
Part H covers drainage: new bathrooms and kitchens must connect to the foul drain with correct falls, and building over or within 3 m of a public sewer requires a Thames Water build-over agreement. Part F requires background trickle ventilation and extract fans in kitchens and bathrooms. Part P requires electrical work in kitchens, bathrooms and outdoors to be carried out and certified by a registered electrician. Part M requires level thresholds where practical and Part K covers stair pitch, headroom and balustrades, which is where many loft stairs fail.
The approval process
There are two routes. A full plans application submits detailed drawings and calculations for approval before work starts, costs £800–£1,500 for a typical extension and gives certainty. A building notice lets work start after 48 hours with inspections on site, but gives no prior approval and is only sensible for small jobs. ICV Buildings always uses full plans for extensions and lofts.
Inspections happen at excavation, foundations, damp-proof course and oversite, drains, structure and insulation, and completion. The completion certificate is the document your solicitor and buyer will ask for, so keep it with the deeds.
Frequently asked questions
- What happens if work does not meet Building Regulations?
- Building control will not issue a completion certificate and can require the work to be opened up or redone. Without a certificate, selling is difficult and buyers usually demand indemnity insurance or a price reduction.
- Do I need an architect for Building Regulations drawings?
- Not necessarily, but you need drawings and specifications detailed enough for the inspector to assess compliance, plus structural calculations. ICV Buildings provides these through our design-and-build service.
- Does Part L apply to the existing house when I extend?
- Only to the new work and any existing elements you alter. You are not required to upgrade the rest of the house, but if you replace windows or the boiler they must meet current standards.