Building Control inspections are one of the most misunderstood parts of any extension. Many homeowners only hear about them once their builder mentions a "BC sign-off" near the end of the project. In reality, Building Control inspections happen at multiple stages during the build — and getting them right is what protects your insurance, your warranty, and your ability to sell the house later.
Who Building Control is
Building Control is a council department (or an Approved Inspector) responsible for enforcing the Building Regulations. They are separate from the planning department — Planning decides whether you can build something; Building Control checks how it is built. The official Building Regulations are published by the Ministry of Housing, Communities and Local Government as Approved Documents A–S, available at https://www.gov.uk/government/collections/approved-documents.
How approval works: Full Plans vs Building Notice
There are two ways to get Building Regulations approval:
- Full Plans application: drawings and calculations submitted before work starts. Approved or rejected within 5–8 weeks. Provides certainty before construction.
- Building Notice: no advance approval — Building Control inspects during the build. Faster to start but riskier, as issues identified mid-build can require rework.
For any substantial extension, we always recommend Full Plans. The £100–£200 extra cost is well worth the certainty.
The 6 key inspection stages
On a typical extension, Building Control inspects at these milestones. The builder calls in the inspector at each stage; the inspector either signs off or specifies remedial work. Work should not progress past an inspection point until sign-off is received.
1. Commencement (week 1)
Notification that work has begun. Often combined with the first physical inspection. No physical check at this stage — it triggers Building Control to open the file and schedule subsequent inspections.
2. Foundations (week 2)
Inspector checks the foundation trenches before concrete is poured. They verify:
- Trench depth meets the structural design (usually 1m on standard ground)
- Trench width is sufficient for the wall load above
- Ground conditions match the engineer's assumptions
- Reinforcement (if required) is correctly placed
If anything is wrong, it is far cheaper to fix now than after pouring concrete. We never start a pour without sign-off.
3. DPC and drainage (week 2–3)
Check of damp-proof course position and drainage runs before the building is sealed. Inspector verifies:
- DPC is at the right height (150mm above external ground level)
- Drainage pipes are at the correct fall (typically 1:40)
- Drainage connections to the existing system are sound
- Any drainage diversions have been properly handled
4. Structural and weatherproof stage (week 4–5)
Major check before the structure is closed in. Inspector verifies:
- Steel beams (RSJs) are the correct size and properly seated on padstones
- Lintels above doors and windows meet the structural design
- Wall ties are correctly spaced
- Cavity insulation is properly installed with no cold bridging
- Roof structure is correctly built (rafters, ridge, eaves detail)
5. Insulation and pre-plaster (week 6–7)
After first fix but before plasterboarding closes everything up. Inspector verifies:
- Wall, floor, and roof insulation U-values meet current standards
- Sound deadening where required (e.g. between dwellings on terraces)
- Fire stopping in cavities and party walls
- Ventilation provisions (trickle vents, extractor fans)
- Electrical fire protection (Part P compliance)
6. Completion (week 11–12)
Final inspection once all work is complete. Inspector verifies the finished build matches the approved drawings and issues a Completion Certificate.
Why the Completion Certificate matters
The Completion Certificate is the most important document from your extension project. It confirms the build was inspected and meets the Building Regulations. You will need it:
- When selling the property — solicitors always ask for it during conveyancing
- For insurance — some insurers require it on the policy renewal after a build
- For mortgage applications by future buyers
- For warranty claims if anything goes wrong with the work
Without a Completion Certificate, an extension is treated as "unauthorised" — the council can serve enforcement notices, and a future sale becomes substantially more complicated and expensive. We never leave a project without one in place.
What if the inspector finds problems?
Building Control inspectors are typically pragmatic. If they find a problem they will specify what needs to change and re-inspect. On a properly managed project this is rare — but when it happens, the cost falls on the builder, not the homeowner, provided you have a written contract specifying Building Regulations compliance. This is one reason a proper itemised quote with clear terms matters.
We manage every Building Control inspection for you
JCT Building handles all Building Control coordination — applications, inspection bookings, sign-offs, and the final Completion Certificate. You receive the certificate at handover. Call 07769 225120 or use the contact form to book a free site visit.
