Building Inspection Stages: What Gets Checked and When

Once you have building regulations approval and work starts on site, the building control officer (BCO) becomes a periodic but important presence. Their role is to inspect the construction at key stages and confirm it complies with the approved plans and the relevant Approved Documents. Understanding what they check at each stage, and why, makes you a better-informed client and helps you hold your builder accountable.

The inspection regime is not a formality. The BCO is an independent professional whose sign-off at each stage is what eventually leads to a completion certificate. They have the authority to require work to be opened up if it's been covered before inspection, and in serious cases they can require demolition and rebuilding. Neither outcome is desirable. The simple way to avoid both is to ensure your builder notifies building control before covering any stage.

Commencement Notification

Before work starts, the BCO (or Approved Inspector) must be notified that work is commencing. This is the building notice equivalent of switching the clock on. Without commencement notice, your building regulations application lapses after three years.

For most domestic projects, 48 hours' notice before commencement is required. Your builder should handle this, but check it's been done. Ask them to send you confirmation or a copy of the notification.

Foundation Excavation

This is the first critical inspection. Before any concrete is poured into the foundation trenches, the BCO must see the excavated ground. They're checking that the foundations are being built to the design specification and that the ground conditions match the designer's assumptions.

Ground conditions often differ from what was assumed at design stage. A BCO may find that the trenches need to be deeper than designed (to reach adequate bearing capacity), that the soil is more variable than expected, or that there are tree roots, services, or fill material requiring a different approach. If concrete is already in the trench when the BCO arrives, none of this can be assessed.

The most common missed inspection on domestic projects is the foundation excavation. Builders push to pour concrete because it lets them move on quickly. Don't let your builder pour before the BCO has signed off. Period.

Oversite and Damp Proof Course

Before the floor slab (oversite concrete) is cast, the BCO checks the hardcore bed, any insulation, the radon membrane (if required by the site's radon mapping), and the arrangement of services passing through or under the slab. Once concrete is poured, none of this is accessible without breaking out the floor.

The damp proof course (DPC) inspection confirms the DPC is correctly positioned in the external walls, typically two courses above external ground level, and is correctly lapped with the damp proof membrane in the floor. Bridging the DPC (allowing render or debris to accumulate above it on the external face of the wall) allows moisture to bypass it. The BCO will check this is not happening.

Structural Elements

Any structural steelwork, timber frame elements, or engineered timber beams must be inspected before they're concealed within walls, floors or ceilings. The BCO checks that the specified sizes and grades of material have been used, that connections are correctly made, and that fire protection (intumescent paint or boards) has been applied where required.

For extensions that involve a steel beam spanning across a new opening, the BCO will want to see the steel in position before plasterboard goes up. For loft conversions, the structural floor and new roof structure will be checked before boarding or insulation is installed.

Drains

Before drainage trenches are backfilled, the BCO inspects the drain runs for correct fall, correct jointing, and correct connection to the existing system. A drain test (air or water pressure test) is carried out at this stage to confirm the runs are watertight.

Drainage failures are expensive to remedy because the drains are buried. A missed drain inspection that results in an inadequately laid run can cause years of problems: slow drainage, smells, subsidence as water erodes the surrounding soil, and eventual collapse. The inspection takes 20 minutes. Getting it signed off is worth the wait.

Insulation and Airtightness

Part L (energy efficiency) requires specific U-values for walls, roofs and floors in new build and extension work. The BCO will check that the specified insulation has been used and correctly installed. For wall insulation, this inspection needs to happen before the inner skin of blockwork is built (in a cavity wall) or before internal finishes are applied.

Airtightness is increasingly important following the 2021 Part L revisions. For some project types, an air pressure test may be required at completion. This involves pressurising the building and measuring air leakage. If the result doesn't meet the required standard, you'll need to identify and seal the leakage points. Getting the insulation and membranes right during construction is much easier than trying to improve airtightness retrospectively.

Fire Safety

Part B covers fire safety. For an extension, the BCO will check that fire-resistant doors are fitted where required (between garage and house, for example), that fire alarms comply with the current standard (mains-wired with battery backup, interlinked throughout the affected areas), and that any openings in fire-resisting construction are correctly fire-stopped.

Loft conversions in particular have demanding fire safety requirements. New habitable rooms in loft conversions require a protected escape route to the final exit. In most cases this means fire doors to all rooms on the floor below, and in some cases sprinklers or mains-wired fire detection. The BCO will specify what's required based on the design.

Final Inspection and Completion Certificate

When the building work is substantially complete, the BCO carries out a final inspection of the whole project. This covers:

• Confirmation of all structural and fabric elements previously inspected
• Compliance of sanitary fittings and plumbing (Part G)
• Ventilation provisions (Part F): trickle vents in windows, extract fans in bathrooms and kitchens
• Electrical installation (they won't test it themselves, but will ask for the electrician's certification)
• Glazing safety: low-level glazing in areas where impact is likely must be safety glass (Part N)
• Stair design: headroom, tread dimensions, handrail requirements (Part K)
• Access: level thresholds, door widths where required (Part M)

Once satisfied, the BCO issues the completion certificate. This is the document that matters: it confirms the work was built to regulations. Keep it with your title deeds and property documents. Produce it if you ever sell or remortgage.

What Happens with Missed Inspections

If your builder fails to notify building control at a required stage and the work is covered up, the BCO has three options: accept a description of the work based on photos and calculations, require the work to be opened up for inspection, or refuse to issue a completion certificate.

Opening up work is at the building owner's cost. If the BCO requires foundations to be exposed after backfilling, that means excavating again around the slab. If the structural steel was covered without inspection, it means cutting into finished ceilings or walls. These are expensive, disruptive outcomes that are entirely avoidable.

Build a simple checklist of the required notification stages for your project and mark each one off as it's completed. It takes ten minutes to set up and can prevent a very costly mistake.