Council building control versus approved inspectors
Building control in London can be delivered by either the borough's own building control team or a private Approved Inspector — both routes carry equal legal weight under the Building Act 1984. The borough route involves submitting either a full plans application or a building notice; the approved inspector route involves serving an Initial Notice on the council, after which the inspector takes over statutory duties. Around 60% of London residential work in 2026 uses approved inspectors because they offer faster response, longer site visit slots, and continuity with a named inspector. After the Building Safety Act 2022, all building control work is now overseen by the Building Safety Regulator, and inspectors must be registered as Registered Building Control Approvers (RBCAs).
Stage one — foundations and excavation
The first statutory inspection is at excavation, before concrete is poured. The inspector checks the depth and width of the trench against the structural design, the bearing soil (London clay typically requires 1m minimum depth to avoid seasonal heave, deeper near mature trees), the presence of any drains or services, and that the formwork matches drawings. In areas with heave-susceptible clay and nearby trees, the depth may need to be 2.5m or more — Wandsworth, Lambeth and Lewisham have notorious clay-with-tree conditions where heave-resistant foundations are routine. After inspection the inspector signs off pour authorisation; pouring concrete before inspection is a breach and the inspector may require excavation to verify, at significant cost.
Stage two — damp proof course (DPC) and oversite
Once foundations cure and the base brickwork rises to DPC level, the inspector checks the DPC material (typically a 1200-gauge polyethylene membrane or pitch polymer sheet), continuity with the floor DPM, height above ground level (minimum 150mm), and the oversite concrete or beam-and-block floor. For new builds, the radon barrier and ground gas membrane (if required by site assessment) are inspected at this stage. London boroughs with elevated radon (limited but present in some south and west pockets) require a radon barrier; gas membranes are required on brownfield and former landfill sites across east London.
Stage three — drainage
Below-ground drainage is inspected before backfilling — once trenches are closed, defects are very expensive to find. The inspector checks pipe falls (minimum 1:80 for foul, 1:100 for surface water on 100mm pipes), bedding (pea shingle or class A material), connections to manholes, manhole construction, and the connection to the public sewer (where Thames Water has separate approval requirements via the Section 106 process). A pressure or water test may be required — the contractor blocks the line and fills with water; any drop indicates a leak. Post-Approval rectification of drainage defects has been a sustained problem in London — inspectors are now requiring photo evidence at every joint before backfill.
Stage four — structure, before plastering
The structural inspection happens once the frame is up and the roof on, before plasterboarding closes the walls. The inspector checks steel beam sizes and end bearings against the engineer's calculations, lintels, padstones, fixing of timber joists into hangers or wallplates, the wallplate strap density, restraint straps at gables, and any timber treatment. This is the stage where structural engineer involvement is essential — most inspectors will require a sign-off letter from the engineer confirming steelwork is installed per drawings. After this inspection, plasterboarding can proceed.
Stage five — insulation, fire protection, and services
Before plastering, the inspector checks thermal insulation against the Part L design (typically 100–150mm PIR in walls and 270mm mineral wool in roofs for 2026 standards), continuity of the air barrier, fire-stopping at service penetrations, and Part P electrical compliance (the electrician must be Part P registered and provide a certificate). For loft conversions, fire-rated doors to all habitable rooms on the escape route, mains-wired interlinked smoke alarms, and fire-protected partitions to BS EN 13501 standards are checked. Mistakes at this stage are expensive — failed air pressure tests often require plasterboard to come down and the air barrier to be repaired.
Final inspection and completion certificate
The final inspection is comprehensive: it covers everything not previously sign-off and the overall standard of the completed work. The inspector checks final electrical installation certificates, gas safety certificate (Gas Safe registered installer), MVHR commissioning records if installed, air pressure test results (Part L compliance), water efficiency calculation (110 litres per person per day), and ventilation rates. If everything is in order, the Final Certificate (or Completion Certificate from an approved inspector) is issued within 8 weeks. This is the document mortgage lenders and buyers' solicitors require — without it, the property is hard to sell or remortgage. Keep it permanently with the title deeds.
