Building Control Site Inspections: What Happens at Each Stage of Your Extension

**Overview — when and why inspections happen**:

• Building Control inspections are not a single end-of-project check. They are staged inspections throughout the construction process — each one checking the works that are visible at that stage, before they are covered by the next element of construction. This staged approach exists because:
• •Many structural and technical elements (foundations, drainage, insulation, structural steelwork) are permanently concealed during construction and cannot be checked retrospectively without destructive investigation
• •The contractor must demonstrate at each stage that the works meet the relevant Building Regulations requirements before proceeding
• •If a compliance failure is identified at inspection, it can be rectified before the next element covers it — avoiding the much greater cost and disruption of opening up completed works

**Who to notify before each inspection**:

For Local Authority Building Control (LABC), inspections are typically booked via the council's online portal or by phone, with at least 48 hours' notice before the inspection is needed. For Registered Building Control Approvers (private approved inspectors), the process varies — check the specific Approver's booking system. The contractor is typically responsible for booking inspections at the appropriate stage.

**Stage 1 — Commencement notification (before work starts)**:

The contractor must notify Building Control before work commences — typically at least 48 hours before the start on site. This is a legal requirement under the Building Regulations (Regulation 16). For a Full Plans application: the approval is in place before commencement notification is given. For a Building Notice application: the notice IS the commencement notification (typically given when the notice is submitted, not at a separate later step).

At commencement, Building Control may or may not visit — this is a notification, not necessarily a site inspection. Some Building Control offices conduct a first site visit at commencement; others wait until the excavation stage.

**Stage 2 — Excavations (foundation trench) inspection**:

The excavation inspection is one of the most important inspections for a London rear extension. The inspector attends when the foundation trenches have been excavated but BEFORE the concrete is poured.

• *What the inspector checks at excavations*:
• •Depth of the trench (to confirm bearing strata — for a typical London extension on London Clay, foundations should typically be at 1.0–1.5m minimum depth, deeper near trees due to clay shrinkage/heave risk)
• •Width of the trench (to confirm adequate bearing area for the foundation concrete)
• •Condition of the bottom of the trench (firm, undisturbed ground — 'formation level' — free from soft spots, tree roots, or made ground)
• •Position of the trench relative to any drainage runs, trees, or existing structures (to check for encroachment or inadequate clearance)

*What happens if the ground is not at the right depth or condition*: The inspector will instruct a deeper dig, additional trench width, or in some cases a change to a different foundation type (piled foundations where bearing conditions are poor). The concrete must not be poured until the inspector has approved the excavations.

*Timing note*: In London, particularly near trees on London Clay, Building Control may require soil samples or tree root assessments before approving the excavations at a standard depth. Plan for this possibility in areas with significant tree coverage.

**Stage 3 — Drainage inspection (before backfill)**:

• All drainage runs installed in the ground must be inspected and tested before being backfilled. The inspection checks:
• •Position of drain runs (matching the drainage plan)
• •Falls (gradients — measured with a level; foul drainage minimum 1:80 for 110mm pipe, typically around 1:60)
• •Jointing and connections (correctly jointed to inspection chambers, gullies, and existing runs)
• •Access to all new runs via inspection chambers (IC positions per plan)
• •Air or water pressure test: the inspector may require a pressure test of all new drain runs — typically an air test at 100mm water gauge for 5 minutes. The test confirms all joints are watertight.

*Timing note*: This inspection must happen BEFORE the drainage is buried (before the floor slab is poured over in-ground drainage and before backfilling of external drainage runs). Booking this inspection too late — after the slab is poured — means the drains cannot be inspected and the inspector may require opening up.

**Stage 4 — Damp-proof course (DPC) level inspection**:

• The DPC inspection takes place when the new walls have risen to DPC level (typically at the first course of brickwork above the concrete foundation, or at 150mm above external ground level). The inspector checks:
• •DPC is present and correctly positioned (a continuous horizontal layer of impermeable material — typically 150 micron polythene or engineering brick — at 150mm minimum above the external ground level)
• •The DPC relates correctly to the DPM (damp-proof membrane in the floor)
• •Where the extension wall returns to the existing wall, DPC is continuous and connected to the existing DPC
• •Cavity closers are in place where the cavity terminates at openings (door and window frames) — vertical DPC at all reveals

*Why this inspection matters*: Rising damp in an extension is almost always caused by an incorrectly positioned or discontinuous DPC. This inspection confirms the DPC is correct before the walls are built up above it and the floor concrete is poured.

**Stage 5 — Concrete floor slab or beam and block floor inspection**:

• Where the extension has a concrete ground floor slab, an inspection may be required before the slab is poured (to confirm the floor build-up layers are correct). The inspector checks:
• •DPM (damp-proof membrane) is correctly positioned and lapped up at the edges to join the DPC
• •Floor insulation type and thickness matches the specification (confirming Part L compliance — typically 100mm PIR minimum)
• •Where UFH is installed, pipes are in place and pressure-tested before the screed is poured
• •Formation level is correct (sub-base compacted, no soft spots in the ground)

Note: in some cases, the slab inspection is combined with the DPC inspection — check with your specific Building Control office.

**Stage 6 — Structural steelwork inspection**:

• Where structural steelwork is installed (typically the steel beam over the new structural opening from the existing house into the extension), an inspection is conducted to check:
• •Beam is the correct section (matching the SE's specification — note the universal beam designation: e.g., 203×133 UB 25)
• •Beam is correctly bedded on padstones at each bearing (padstone size, material, and bedding as per SE's specification)
• •Beam is at the correct level (supporting the structure above correctly)
• •Temporary propping is in place where required before the beam installation and has been correctly removed after

The structural steel inspection may be combined with a general structural inspection (checking all visible structural elements: new masonry walls, lintels over openings, structural opening formation).

**Stage 7 — Roof structure and insulation inspection**:

• For flat roof extensions, an inspection at roof structure and insulation stage may be conducted before the waterproofing membrane is applied. The inspector checks:
• •Structural deck (OSB3 18mm — type and thickness as specification)
• •Falls on the deck (minimum 1:80 to drainage outlets — measured with a straightedge and level)
• •Insulation type and thickness (matching specification — minimum 150mm PIR for post-2022 Part L compliance; see `building-regulations-part-l-guide`)
• •VCL (vapour control layer) position (should be on the warm side of the insulation in a warm flat roof — directly above the ceiling)
• •Rooflight or drainage outlet positions

• For pitched roof extensions (loft conversions, pitched-roof single-storey extensions), the inspection checks:
• •Rafter sizes and spans (matching SE's design)
• •Ridge beam (if new structural ridge is installed)
• •Insulation between and over rafters (for loft conversions: typically between-rafter and over-rafter combined to achieve U-value ≤ 0.15 W/m²K)
• •Ventilation to any cold roof void (where insulation is between rafters only)

**Stage 8 — First-fix electrical inspection (Part P)**:

• For new electrical circuits (ring mains, lighting circuits, extract fan circuits), Building Control under Part P of the Building Regulations requires either:
• •The work to be carried out by a competent person registered with a Part P competent person scheme (NICEIC, NAPIT, ELECSA or similar) — who can self-certify compliance and issue the Electrical Installation Certificate (EIC) directly, notifying Building Control automatically; OR
• •For non-registered electricians: formal Building Control notification, including a Part P application, and inspection by an approved inspector

In practice, most London residential electrical contractors are NICEIC or NAPIT registered — they self-certify and the EIC is issued to the homeowner directly. Building Control sees a copy of the certificate. The homeowner should receive the original EIC on completion.

**Stage 9 — Wall insulation/window installation inspection**:

• Some Building Control offices conduct a mid-construction inspection when wall insulation is installed (before the inner leaf of cavity masonry is laid) or when windows and external doors are installed. This checks:
• •Cavity insulation (product, thickness, continuity — no gaps in full-fill insulation) before the inner leaf covers it
• •Window and door frame installation (sealed reveals, damp-proof closure at sills, trickle ventilators fitted as required by Part F)

**Stage 10 — Occupation (pre-occupation) inspection**:

• For loft conversions or projects with significant fire safety requirements, Building Control may require an inspection before occupation of the new space. This checks:
• •All fire doors are installed, self-closing, and correctly fitted
• •All smoke alarms are installed, interlinked, and tested
• •Escape routes are clear and comply with Approved Document B requirements
• •Any sprinkler system (where required) has been commissioned

**Stage 11 — Final inspection and completion certificate**:

• The final inspection takes place when all works are substantially complete. The inspector checks:
• •All relevant conditions of the Full Plans approval have been met
• •The works as built match the approved drawings
• •All compliance certificates have been received: Gas Safe certificate; Electrical Installation Certificate; drainage test report; any specialist materials certificates (e.g., membrane installer's guarantee, insulation data sheets)
• •Structural completion sign-off has been received from the SE (where required by the approval condition)

• On satisfactory completion of the final inspection, Building Control issues a **Completion Certificate** — the formal written confirmation that the works comply with the Building Regulations. This is an important document:
• •Keep the original Completion Certificate permanently with the property deeds
• •Conveyancers (solicitors) will ask for the Completion Certificate when the property is sold
• •Mortgage lenders may require the Completion Certificate before releasing funds on a remortgage
• •Insurance companies may require it for rebuilding or extension-related claims

**What happens if the final inspection finds compliance failures?**

• If the final inspection identifies compliance failures:
• •Minor issues: Building Control may issue a 'snagging list' of minor items to be rectified before the Completion Certificate is issued. These are typically items that are straightforward to fix (a missing smoke alarm, an incomplete fire door fitting, a silicone seal gap).
• •Significant issues: Building Control may require more substantial remediation before the Completion Certificate is issued. If the failure is structural (e.g., insulation thickness insufficient — discovered because the inspector can see the roof spec does not match the design), opening-up works may be required.
• •Enforcement: In rare cases where works are not remediated, Building Control can serve a Contravention Notice requiring the homeowner to carry out specific remediation work within a time limit, or can require opening up of specific elements for inspection.

**Time limits for Building Control applications and inspections**:

• Building Regulations applications do not have an expiry date for the approval itself (unlike planning permission). However:
• •The Building Regulations apply for the duration of the project
• •The Final Inspection and Completion Certificate request must be made within 8 years of the date of the original Full Plans application (in practice, for most residential projects this is not a concern)
• •If a project is started but not completed (e.g., financial difficulties, abandonment), the Building Regulations application remains live and the completion obligation persists

**Who is responsible for booking Building Control inspections?**

• In a standard residential building contract, the principal contractor is responsible for notifying Building Control at each required inspection stage and for managing the inspection process. The homeowner should confirm this is the case in the contract — particularly:
• •That the contractor will give adequate notice to Building Control for each stage
• •That the contractor will not proceed to pour concrete or cover drainage or insulation before the inspector has attended
• •That the contractor will promptly notify the homeowner of any inspection outcomes, including any compliance issues identified

**What the homeowner should do to protect themselves**:

1. *Ask for the inspection record*: Request a copy of all Building Control inspection reports (or digital confirmations from LABC online portals). Keep a file of all inspection pass-offs and any notes from the inspector.

2. *Do not allow the contractor to proceed without inspection*: If the contractor proposes to pour the concrete without waiting for the Building Control excavation inspection, or to backfill drainage without a drainage inspection, this is a red flag. Insist on waiting for inspection. A compliant contractor will never resist this.

3. *Book the final inspection in good time*: The final inspection should be booked when all works are substantially complete and before any snag list is addressed (so the inspector sees the project in its near-complete state). Do not occupy the new space before the final inspection and Completion Certificate are obtained.

4. *Keep the Completion Certificate*: The Completion Certificate is a permanent legal document. Store it with the property deeds and provide it to your solicitor at the point of any future sale.

5. *Chase an overdue Completion Certificate*: After the final inspection, if the Completion Certificate has not been issued within 8 weeks, chase Building Control. An overdue Completion Certificate can become a problem in a property sale if the solicitor's search reveals an open Building Regulations application.

**Regularisation — what if Building Control inspections were missed?**

• If works have been carried out without Building Regulations approval or without the required inspections, a Regularisation Certificate can be applied for retrospectively. Regularisation involves:
• •Applying to Building Control with details of the works as built (including any available drawings or specifications)
• •Building Control inspecting the works — which may require opening up sections to check concealed elements (foundations, drainage, insulation)
• •Building Control confirming whether the works comply with the Building Regulations at the time the works were carried out
• •Issuing a Regularisation Certificate if compliance is confirmed (or requiring remediation if not)

Regularisation is more expensive and disruptive than proper initial compliance — the inspection may require significant opening up to check concealed structural elements. It is also not available in all situations — for example, Building Control may decline to regularise works that are clearly non-compliant and require demolition and rebuilding.

For works that are over 12 months old and where there was no enforcement action, Building Regulations enforcement powers expire (though the Completion Certificate is still needed for property conveyancing). In this situation, building regulations indemnity insurance (obtainable from specialist brokers) may be an alternative to Regularisation in some property sale situations — the insurance protects the purchaser against the cost of any enforcement action by the local authority. A solicitor should advise on whether indemnity insurance or Regularisation is the better approach for a specific property situation.