Building Regulations for Extensions, Lofts and Basements: What Is Checked and When
Every extension, loft or basement conversion needs Building Regulations approval, whatever the planning position. Which Approved Documents apply, full plans versus building notice, the inspection stages and the completion certificate.
Every extension, loft conversion and basement conversion in England needs Building Regulations approval, whether or not it needs planning permission. The Building Regulations 2010 set minimum standards for structure, fire safety, insulation, ventilation, drainage, electrics, glazing, stairs and access, explained in the government's Approved Documents A to T. Approval is given by a building control body — the council or a private registered building control approver — which checks the design, inspects the work at set stages and issues a completion certificate that your solicitor will ask for when you sell. This guide explains which parts apply to which projects, the two ways to apply, and what happens on site.
Planning permission and Building Regulations are different things
Planning is about whether a building may exist — its size, appearance and effect on neighbours. Building Regulations are about whether it is built safely and efficiently. A rear extension can be permitted development (no planning application) and still need a full Building Regulations application; a listed-building alteration can have planning and listed building consent and still fail building control on fire safety. The two systems have different applications, different fees and different inspectors. Our guide to planning permission for extensions covers the planning side.
Which Approved Documents apply to your project
| Approved Document | Covers | Extension | Loft | Basement | Renovation |
|---|---|---|---|---|---|
| A — Structure | Foundations, beams, floors, walls, roof; engineer's calculations | ✓ | ✓ | ✓ | Openings, chimney breasts |
| B — Fire safety | Escape routes, fire doors, alarms, fire resistance | Alarms, escape from inner rooms | ✓ protected stair | ✓ escape from basement | Where layout changes |
| C — Site preparation, damp | Ground, contamination, damp-proofing, waterproofing | ✓ | — | ✓ (BS 8102) | Damp works |
| F — Ventilation | Background and extract ventilation | ✓ | ✓ | ✓ | New bathrooms/kitchens |
| H — Drainage | Foul and surface water, build-over of sewers | ✓ | New bathroom | ✓ pumps | New bathrooms |
| J — Heat-producing appliances | Boilers, flues, stoves | Boiler moves | Flue routes | — | New boiler |
| K — Stairs, falls, impact | Stair pitch, headroom, guarding, glazing safety | Glazing | ✓ new stair | ✓ new stair | Stairs, balustrades |
| L — Conservation of fuel and power | Insulation U-values, glazing limits, heating efficiency | ✓ | ✓ | ✓ | Thermal elements, windows |
| M — Access | Accessible entrance, WC provision | Where relevant | — | — | — |
| O — Overheating | Solar gain and ventilation (new dwellings) | New builds only, but good practice for glazed extensions | — | — | — |
| P — Electrical safety | Domestic electrical work | ✓ | ✓ | ✓ | ✓ rewire |
| Regulation 7 | Materials and workmanship | All projects | |||
What building control actually checks on the common projects
Rear or side extension
Foundation depth and width (deeper in clay and near trees — the foundation-depth tables are applied strictly in Barnet and Harrow); damp-proof course and membrane; the steel beam and its bearings; wall, floor and roof insulation to Part L (as a guide, a wall U-value of 0.18 W/m²K and roof 0.15 W/m²K for new elements); the area of glazing relative to floor area (over 25% needs compensating measures or a calculation); drainage runs, gradients and any build-over; ventilation; electrics certified by a Part P registered electrician; smoke alarms where an inner room is created.
Loft conversion
New floor joists — the existing ceiling joists are not designed to carry a floor — and the steels or load-bearing walls that support them; the dormer structure; the stair (maximum pitch 42°, minimum 2 m headroom, 1.9 m/1.8 m at the centre and edge of a loft stair); the fire strategy for a three-storey house: FD30 doors to rooms off the stair on every floor, mains-linked smoke alarms on every level, and a protected route to the outside or an escape window; roof and dormer insulation; the new bathroom's drainage and ventilation. The Planning Portal's loft fire-safety guidance summarises the fire requirements.
Basement conversion
Underpinning sequence and temporary works, inspected bay by bay before backfilling; the waterproofing design (BS 8102:2022 Type A, B or C, usually a combination); drainage with a sump and pump where gravity drainage is not possible; means of escape — a protected stair or an escape window/door into a light well; ventilation and insulation; structural checks on the walls above.
Whole-house renovation
Structural openings and chimney-breast removal; electrical rewiring (Part P); new bathrooms and their drainage and extract ventilation; replacement windows (through FENSA or building control) and thermal upgrades where more than 25% of an element is renewed; new boilers and flues (Gas Safe registered installers self-certify).
Full plans or building notice?
Full plans: you submit drawings, specifications and structural calculations; building control checks them and issues an approval (or conditions) before or shortly after work starts, then inspects on site. It takes five to eight weeks and costs more up front, but the design is approved in advance, which is what a fixed-price contract needs. Building notice: you give 48 hours' notice and start; compliance is checked on site as the work proceeds. It is faster and suits small, simple jobs, but anything structural risks being told to change it after it is built. We use full plans for every extension, loft and basement.
The inspection stages
- Commencement — notice before work starts.
- Excavation — trench depth, ground and trees checked before concrete is poured.
- Foundations — concrete and any reinforcement.
- Damp-proof course and oversite — before the ground floor is laid.
- Drainage — pipes laid and tested before backfilling.
- Structure — steels, bearings, floor and roof structure before they are covered.
- Insulation — before plasterboard.
- Completion — fire doors, alarms, electrical and gas certificates, glazing, ventilation; the completion certificate follows.
We photograph every stage before it is closed in and keep the record with the project file, so there is evidence of what is behind the plaster long after the inspector has gone.
Fees
Building control fees are set by each authority and by private approvers; for a typical extension or loft conversion in North London expect £900–£1,800 for the full plans application and inspections combined, and £1,500–£3,000 for a basement. Structural engineer's calculations are separate (£900–£2,500). Regularisation of unapproved work costs more than a fresh application and involves opening up.
Completion certificate, warranties and selling
The completion certificate is the document a buyer's solicitor will ask for. Keep it with the electrical installation certificate, the Gas Safe certificate, the FENSA certificate for windows, the party wall award if there was one and any product guarantees. For new-build houses lenders will also require a 10-year structural warranty from a provider such as LABC Warranty; for extensions and conversions warranties are optional and worth discussing before work starts.
How we handle it
Building Regulations drawings, structural calculations and the full plans submission are part of the design stage on every Milkov & Son project, and the site programme is built around the inspection stages. See architectural design and planning and how a project runs.
Sources
Frequently asked questions
Do I need Building Regulations approval if I don't need planning permission?
Yes. Planning and Building Regulations are separate systems. Permitted development removes the need for a planning application; it has no effect on Building Regulations, which apply to every extension, loft and basement conversion and to structural, electrical, drainage, glazing and thermal work in a renovation.
What is the difference between full plans and a building notice?
A full plans application submits drawings and calculations for approval before work starts, so the design is checked and approved in advance. A building notice lets you start after 48 hours' notice but leaves compliance to be assessed on site as work proceeds. For anything structural we use full plans.
Who inspects the work?
Either the local authority's building control team or a private registered building control approver (formerly 'approved inspector'). Both work to the same regulations and issue a completion certificate at the end.
What happens if work was done without Building Regulations approval?
The council can require it to be opened up, altered or removed, and a buyer's solicitor will ask for the completion certificate when you sell. Regularisation — retrospective approval — is possible but involves exposing the work for inspection and costs more than getting approval first time.
Does a loft conversion need fire doors?
Converting a two-storey house to three storeys normally requires a protected stair enclosure: FD30 fire doors to the rooms opening onto the stair on all floors, mains-wired interlinked smoke alarms on every level, and either a protected route to the front door or an escape window.
About this guide. Written by the Milkov & Son Construction team and reviewed by Stoyan Milkov, Director & Project Manager. Cost figures are North London market guides for the year stated and not quotations; planning and Building Regulations summaries are general and should be confirmed for your property. About the company.
