Building control compliance guide
Building Control for Extensions: What Inspectors Want to See
Plain-English guide to the submission routes, the inspection stages, the documents inspectors read first, and the six rejection reasons we see most often on UK domestic extensions.
TL;DR
Before you lay a single brick on an extension you need building control approval — separate from planning. This guide walks through the two submission routes (Full Plans vs Building Notice), the five inspection stages, the documents inspectors actually read, and the six rejection reasons we see most often so you can avoid them on your next extension.
In this guide
When Building Control Is Required
Almost every domestic extension in England and Wales needs building control approval. This is completely separate from planning permission, and getting one does not mean you have the other. Under the Building Regulations 2010 the local authority (or a private Approved Inspector acting on their behalf) has to sign off the structural, thermal, fire, drainage, and accessibility elements of any new building work that alters or adds to a dwelling. That includes single-storey rear extensions, side returns, two-storey extensions, loft conversions, garage conversions, basement conversions, and almost every porch over 30m².
The common exemptions are narrow. Conservatories under 30m² built at ground level with their own independent heating controls and thermally-separated from the main house are exempt. Porches under 30m² at ground level with an external-quality door between the porch and the main house are exempt. Detached outbuildings under 30m² with no sleeping accommodation are exempt. Everything else needs approval.
If you are the contractor doing the work, you also have overlapping duties under CDM 2015 to plan, manage, and monitor the work safely — which in practice means a site-specific RAMS and, for notifiable projects, a Construction Phase Plan. Building control inspectors almost always ask to see both documents on first site visit, even though CDM is technically HSE’s jurisdiction, not theirs.
The test is simple: if the work affects structure, thermal performance, fire safety, ventilation, drainage, or accessibility, building control applies. If you are unsure, phone the duty officer at your local authority before you do anything else — every council publishes a free duty officer line and a two-minute call will tell you definitively whether approval is needed.
Step-by-Step Submission Process
There are two parallel routes into building control approval for a domestic extension: Full Plans application and Building Notice. They achieve the same end result (a completion certificate) but work very differently in practice, and the choice matters for the builder’s risk exposure and the client’s programme.
The Full Plans application is the safer route for anything technically unusual. You submit detailed drawings, structural engineer’s calculations, SAP or SBEM thermal assessments, specification schedules, and a drainage plan. The plan-check surveyor reviews the package and either approves unconditionally, approves with conditions, or rejects with a list of required amendments. Legal determination must happen within five weeks, though in practice most domestic extensions clear in three to four. Once approved, you are cleared to start and the technical risk sits with the surveyor — if the plans were approved, they cannot later require you to change work that matched the approved drawings.
The Building Notice is a simpler two-page form that lets you start within 48 hours of submission. There is no up-front plans review, which means faster programme and lower plan-check fees, but all the technical risk sits with you at site-inspection time. If the inspector arrives at foundation stage and decides the trench is in the wrong position, you dig it again. If the structural arrangement won’t meet Part A, you change it mid-build. Building Notices work well for simple like-for-like extensions where the builder has built the same detail many times before, but they are risky for anything involving steel beams, load-bearing alterations, or unusual ground conditions.
Whichever route you pick, also decide early between local authority building control and a private Approved Inspector. Both produce the same legal completion certificate; lenders and buyers treat them identically. Local authority is usually cheaper (no VAT on LA fees), private is usually faster (competitive plan turnaround). For a straightforward rear extension most builders default to LA; for anything with a tight programme or specialist construction, an Approved Inspector often pays for itself in saved weeks.
Whatever you do, keep a paper trail. File the stamped plans, the commencement notice, every inspection slip, and every email from the surveyor. If the job gets rejected or goes to determination, that paper trail is the only thing that protects you. If your paperwork has already been rejected by building control, there’s a sister guide that walks through the fix sequence.
Common Reasons Extensions Get Rejected
After reviewing hundreds of rejected applications and failed inspections, the same six causes come up over and over. Any of them is enough to hold a plans application or require a structure to be opened up at site stage.
1. Missing or vague structural calculations
Part A (Structure) is the most common plans-stage rejection. An architect drawing a 152x152 UC beam without calculations is not enough; the surveyor needs stamped calculations from a structural engineer showing span, imposed load, bearing length, deflection, and padstone design. Generic beam schedules copied from another job are an automatic reject.
2. Drainage falls below 1:40
Foul and surface water runs have to fall at 1:40 for 100mm pipe, 1:80 for 150mm. Anything less and the run clogs. On-site inspectors check with a spirit level — if the level sits wrong, the section has to be re-dug and re-laid.
3. Cavity closers and fire-stopping gaps
Every opening in a party wall and every cavity at a compartment boundary needs continuous fire-stopping. Missing closers around windows and doors is the most common pre-plaster rejection we see — an inspector will shine a torch down every cavity and the work gets opened up if they find a gap.
4. Thermal insulation below current Part L U-values
Part L 2021 tightened the thermal targets. Walls need 0.18 W/m²K, flat roofs 0.15 W/m²K, ground floors 0.18 W/m²K. Using pre-2021 insulation specifications (common on older drawings) will fail without a written exemption. Check U-values against the current Approved Document L before ordering materials.
5. Lintel bearings under 150mm each side
Minimum end bearing for a steel or concrete lintel is 150mm into solid masonry. We see 100mm bearings on older drawings and on hurried site work constantly — it is a Part A structural rejection every time.
6. No site-specific RAMS on the first visit
Strictly a CDM/HSE matter, not building control, but inspectors check. A generic RAMS downloaded from the internet will not wash — the surveyor wants to see your site address, the specific hazards of this extension, and a named competent person. Generic paperwork is the fastest way to put an inspector in a bad mood on day one.
What Inspectors Check on Site
A domestic extension typically has five site-inspection stages. Each one has a specific trigger and a specific set of things the inspector will want to see. Book each inspection with at least 24 hours’ notice and have the relevant paperwork on site, signed and ready, before the surveyor arrives.
Commencement
Inspector wants the approved plans on site, the commencement notice submitted, the builder’s public liability insurance visible, and on a larger job the CDM paperwork (including your CPP). Takes 20 minutes. Missing the commencement notice means the project is technically still unauthorised.
Foundations (before concrete)
Trenches dug to formation level, spoil pulled back at least 600mm from trench edge, and the ground type visible. Inspector checks bearing soil against the structural engineer’s foundation design. On clay sites they will usually want the trench opened an extra 300mm to confirm shrinkable clay depth.
Damp-proof course and drainage
DPC visible before walls go more than one course above it; drainage runs laid and exposed before backfilling. Inspector spirit-levels the falls, checks cover depth, and often wants to see an air test on foul runs before they are buried.
Structural steel and pre-plaster
Every steel beam exposed with padstones visible and end-bearings measurable. Cavity closers around every opening. Fire-stopping visible at compartment boundaries. Insulation type and thickness visible and matching the approved specification. This is the stage with the highest rejection rate.
Completion
Air-tightness test result, Part P electrical certificate, Gas Safe certificate where applicable, and a final walk-through. On pass, the completion certificate arrives within 10 working days. Missed items at this stage delay sign-off by weeks, not days.
If a stage is missed — for example, foundations poured before inspection — the surveyor can require opening-up work. On a clay foundation already backfilled, that means re-excavating a trench the builder already paid to fill. Book every inspection and keep a site diary of every visit; it is the single cheapest piece of quality control on the job.
Getting Your Completion Certificate
The completion certificate is the single most important document from a building control process. Without it, the extension is legally unauthorised work — not illegal, but a guarantee of solicitor enquiries and mortgage problems at point of sale. With it, the property can be sold, remortgaged, and insured without any awkward questions.
To get the certificate issued, the final inspection has to pass cleanly. That means: the final air-tightness test on target, every Part P electrical certificate filed, the Gas Safe certificate filed if applicable, the thermal envelope commissioning done, and every outstanding defect from earlier inspections closed out. Surveyors typically issue the certificate within 10 working days of the final pass.
Once issued, keep the original with the property deeds. Scan a copy to cloud storage so it is searchable a decade later when the client sells. Give a copy to the client so they have it for their own records. Every UK conveyancing solicitor will ask for a copy during a sale; not having one means the vendor has to buy an indemnity insurance policy or apply for regularisation, both of which cost more than the completion certificate ever did.
For context on how the building control process fits inside the broader UK compliance picture, see our building control compliance hub — it covers the relationship between building control, CDM, and the paperwork trail a domestic client expects on handover. If your paperwork is already in trouble, read RAMS Rejected by Building Control for the remediation sequence.
Planning Permission vs Building Control
Planning and building control are two independent processes that happen in parallel and answer different questions. Planning decides whether you are allowed to build the extension; building control decides whether it is built correctly. You can have planning approval and fail building control. You can build under permitted development (no planning needed) and still have to pass every building control inspection.
A planning officer cares about the external appearance, the neighbour amenity, the overlooking, the scale, and the compliance with local plan policy. They do not care about U-values, lintel bearings, or cavity closers. A building control surveyor cares about structure, fire, thermal, drainage, ventilation, and accessibility. They do not care about external render colour or where the bin store sits.
The practical consequence is that both processes run on different timelines. Planning typically takes eight weeks for a householder application; building control takes three to four for a Full Plans application. Budget for both and do not start on site until you have both clearances (or the Building Notice commencement if you’ve taken that route).
Managing the Client Through the Process
Domestic clients on an extension almost always underestimate how long approvals take. Build the approval timeline into the contract from day one: plans submitted on X, approval expected by Y, works commence on Z. Send the client a scan of the stamped approved plans so they see the process is underway and not stalled.
Explain the difference between the paperwork they need to see — planning decision notice, approved plans, commencement notice, completion certificate — and the paperwork that sits on your side, like the RAMS, CPP, structural calculations, and method statements. Clients value transparency; a one-page “what happens next” handout pre-empts 80% of the anxious phone calls.
Keep the client informed at each inspection stage: booked, done, passed. A photograph of the inspector on site is worth a thousand words; it makes the process real and shows progress. Clients who see regular proof of inspection rarely dispute the final invoice.
What to Do If Work Has Already Started Without Approval
Clients occasionally ask builders to start before plans are approved, and sometimes entire extensions are built without any building control notification at all. If you are mid-project and realise approval is missing, stop, phone the local authority building control duty officer, and submit either a Building Notice or a regularisation application.
A regularisation application is the formal mechanism for getting unauthorised work certified retrospectively. The fees are typically 1.5× the standard charge, and the inspector will almost always require opening-up work so they can see inside the foundations, cavities, and structural elements. On a clay-site foundation, that means digging a sample trench adjacent to the existing footing. Expensive and slow, but infinitely better than no certificate at all.
Never build without approval on advice from a client who thinks building control is optional — the contractor is the one who signs the paperwork and carries the liability. If a domestic client refuses to pay for approval, walk. The mortgage issues and indemnity premiums downstream are not worth the short-term peace.
Local Authority vs Private Approved Inspector for Extensions
Pros
- Local authority inspectors know the area — they’ve seen your exact soil conditions, neighbouring party walls, and drainage layouts before.
- LA fees are usually 15–30% cheaper than private Approved Inspectors for straightforward domestic work.
- Private Approved Inspectors offer faster plan turnaround — often 10 working days vs 3–4 weeks for LA.
- Private inspectors typically assign one dedicated inspector for the whole project — easier continuity than rotating LA duty officers.
- Both routes issue the same legal completion certificate — accepted by mortgage lenders and future buyers with no difference in weight.
- Private Approved Inspectors compete on service, so they’re usually more responsive to booking inspections at short notice.
Cons
- Local authority building control can be slow during peak season — summer extensions sometimes wait 6–8 weeks for plans approval.
- Private Approved Inspectors charge VAT on top; an LA does not, so the net cost gap is larger than the headline quote.
- If a private inspector goes out of business mid-project, the local authority has to take over and may re-inspect work you’ve already had signed off.
- Some rural local authorities only inspect two days a week, forcing you to plan your programme around their availability.
- Not every Approved Inspector covers every postcode — check their geographic range before signing a fee quote.
| Stage | When | What you submit | Who signs off |
|---|---|---|---|
| Initial notice / plans submission | Before you start on site | Full Plans application (drawings + calcs) OR Building Notice (2-page form) | Plan-check surveyor (desk review) |
| Commencement + foundation inspection | Within 48 hours of starting + before concrete is poured | Booked inspection slot; trench dug to formation level with spoil pulled back | Site inspector (visit) |
| Damp-proof course + drainage inspection | Before walls exceed first course above DPC; before trenches are backfilled | Exposed DPC and drainage runs with inspection covers open | Site inspector (visit) |
| Pre-plaster / structural inspection | Before internal finishes cover the structure | Steelwork, fire stopping, insulation, and cavity closers visible | Site inspector (visit) |
| Completion certificate | After final air-test / commissioning paperwork | Air-tightness test, Part P electrical cert, gas safe cert if applicable | Plan-check surveyor (desk review + final visit) |
“The Building Regulations set standards for the design and construction of buildings to ensure the safety and health for people in or about those buildings. They also include requirements to ensure that fuel and power is conserved, and facilities are provided for people, including those with disabilities, to access and move around inside buildings.”
Build the extension paperwork that inspectors accept first time
Inspectors see the same generic RAMS templates every week and reject them every week. The Site Book creates site-specific RAMS and Construction Phase Plans from your extension description — with the exact wording building control and CDM inspectors look for. No more three-round paperwork ping-pong with the surveyor.
See the RAMS Generator →Frequently asked questions
When do I need building control approval for an extension?
You need building control approval for virtually every domestic extension in England and Wales — it is a completely separate requirement from planning permission. Even if your extension falls within permitted development and does not need planning consent, the Building Regulations still apply and building control must sign off the structural, thermal, drainage, and fire-safety elements of the work. The only common exceptions are very small conservatories (under 30m², built at ground level, with independent heating) and detached outbuildings under 30m² that do not contain sleeping accommodation. Porches under 30m² at ground level with external quality doors between the porch and the house are also typically exempt. For a standard single-storey rear extension, a loft conversion, a garage conversion, or any two-storey extension, building control approval is mandatory — no exceptions.
What does building control need to see before I start?
Before work starts, building control needs either a Full Plans application (detailed drawings, structural calculations, SAP/SBEM thermal assessments, and specifications, submitted and approved before you break ground) or a Building Notice (a simpler two-page notification that lets you start within 48 hours but puts all the technical risk on you during inspection). For either route you will also need to submit a site-specific RAMS if you are a contractor working on someone else’s property, a drainage plan showing how foul and surface water will connect, a structural engineer’s calculations for any beams or load-bearing alterations, and evidence that the thermal envelope meets current Part L U-values. Most local authorities also expect to see the builder’s public liability insurance and, for notifiable projects, CDM 2015 paperwork including the Construction Phase Plan.
How long does building control review take for extensions?
A Full Plans application must legally be determined by the local authority within five weeks, extendable to two months by mutual agreement — in practice most straightforward domestic extensions clear in three to four weeks. A Building Notice is effectively instant (you can start 48 hours after submission) but you are building at risk, because the inspector can still require you to open up or remove work during site inspections if anything fails. Private Approved Inspectors typically turn plans around faster than local authority building control — often within 10 working days — because they compete for the work. Foundation inspections once you have started must be booked with 24 hours’ notice, and most inspectors will arrive the next working day. The completion certificate is usually issued within 10 working days of your final inspection, provided no issues remain outstanding.
What's the difference between building control and planning permission?
Planning permission is about whether you are allowed to build the extension — it is decided by the planning department of your local council and focuses on external appearance, neighbour amenity, scale, overlooking, and compliance with local planning policy. Building control is about whether the extension is built safely and to current Building Regulations — it is decided by building control (either local authority or a private Approved Inspector) and focuses on structural integrity, fire safety, thermal performance, ventilation, drainage, and accessibility. The two processes run in parallel and are completely independent. You can have planning permission and fail building control, or build under permitted development (no planning needed) and still be required to meet every Building Regulations standard. Most extension disputes we see are caused by builders confusing the two — a planning officer does not care about U-values, and a building control inspector does not care about the external render colour.
How much does building control cost for a domestic extension?
Local authority building control charges for a domestic extension are set in two parts: a plan check fee (roughly £200–£350 for a single-storey extension up to 40m²) plus an inspection fee (roughly £350–£500). Fees are inclusive of VAT for most local authorities and vary by region — London boroughs tend to charge the highest, rural counties the lowest. A Building Notice typically has a single combined fee of around £500–£700 for the same-size extension. Private Approved Inspectors charge VAT on top and are usually 15–30% more expensive than local authority building control for the same service, but they offer faster turnaround and dedicated inspectors. Additional fees may apply for re-inspections if work fails, and for larger two-storey or loft extensions expect to pay £700–£1,200 in total. Always check your specific local authority website — every council publishes its fee scale.
Related guides
Building Control Compliance Hub
The parent hub covering building control, CDM, and the full UK construction paperwork trail.
RAMS Rejected by Building Control: How to Fix It
If your RAMS has already been rejected, this is the step-by-step remediation sequence.
RAMS for House Extension
A ready-made worked RAMS covering a typical single-storey rear extension.
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Describe the extension in plain English. The Site Book creates a site-specific RAMS and Construction Phase Plan — in the format building control and CDM inspectors actually accept. No credit card required.
Sources
- Approved Document A: Structure — UK Government (MHCLG) · Accessed 20 April 2026
- The Building Regulations 2010 — legislation.gov.uk · Accessed 20 April 2026
- Local Authority Building Control: Services for Homeowners — LABC (Local Authority Building Control) · Accessed 20 April 2026
- HSE — Construction Design and Management Regulations 2015 — HSE · Accessed 20 April 2026
- Planning Portal — Building regulations approval — Planning Portal · Accessed 20 April 2026