What inspections does building control do on an extension in London?

DPC, oversite and drains: the ground-level checks

Once the walls come up out of the ground, the next visit looks at the damp-proof course, the preparation for the new floor and the drains before they are backfilled. Sutton lists new walls dpc and below ground drain pipes for falls and line as one stage, with the damp-proof membrane and floor insulation as the next.

Drains get a second look. The officer usually wants to watch a test of the new runs, and Sutton's schedule asks for drain tests to be witnessed after backfill. Merton tells builders to book the drain inspection before you backfill the drains with draining & connections still exposed. On a terrace with a shared sewer under the garden, the drainage layout in the approved set is what the officer checks the new connection against.

Structure and insulation: the pre-plaster stage

The visit most often missed is the one just before plasterboard goes up. Sutton's schedule has a stage for wall / roof Insulation and roof and floor structure, and Merton's schedule names steel beams and floor, wall and roof timbers as checks of their own. At this point the officer can see the steel sitting on its padstones, the joist and rafter sizes, the insulation thickness and how the new roof meets the old wall.

Each of those is something the building regulations drawings specified: member sizes from the engineer, insulation build-ups on the section, junction details at the abutment. Where what was built matches the sheet, the visit is short. Where the builder has swapped a product or a size, take a revised drawing and, for structure, the engineer's note to the visit, rather than expecting the officer to accept a verbal account.

Knocking through to the house for an open-plan kitchen means a beam; what drawings you need to remove a load-bearing wall covers who prepares each document for that part.

What happens if work is covered up before an inspection

If a stage you were asked to notify is concreted, backfilled or boarded before the officer has seen it, the law gives building control a direct answer. Under regulation 16(6), a person who has not given a required notice must, within a reasonable time, do what the authority asks to cut into, lay open or pull down enough of the work to show whether it complies. In real terms that can mean lifting floor boards, cutting test holes in plasterboard, digging beside a footing or running a camera down a drain.

The cost of opening up and making good falls on the project, not on building control. It also delays the completion certificate, and without one a buyer's solicitor will ask questions later. If an extension was finished with no inspections at all, the route back is a regularisation application; our regularisation cost guide sets out what that involves and our building regulations regularisation service prepares the drawings for it.

The simplest safeguard is a habit: a printed list of the notified stages on site, and a builder who rings building control a day before each one, not after.

The final inspection and the completion certificate

The last visit looks at the finished extension as a home: fire doors and closers where needed, smoke alarms, stairs and handrails, ventilation, headroom and the escape route. Merton lists these as the items usually inspected at completion. Before it issues anything, the building control body will also want the certificates from registered installers for work such as electrics, glazing or a new boiler.

Timescales vary. Sutton says that if all is in order and installer certificates are in, the completion certificate in 5 working days follows the final inspection. GOV.UK's general guidance is that you will usually get a completion certificate within 8 weeks of completion of the building work, as long as it complies.

Keep the certificate with the approved drawings and any revisions issued during the build. The drawings themselves, and what building control reads before work starts, are covered in our companion post on what drawings building control needs for an extension. If an inspection raises a written query about the design, our building control comments service answers it and reissues the sheets for £245; on drawings we prepared, that is free.