Last Updated on August 31, 2026 by Cliche
Building regulation drawings for house extensions are the technical construction drawings a builder and building control need to carry out and check the work — different from planning drawings, which show what the extension will look like and where it sits. Almost every house extension needs them, regardless of whether planning permission was required. Planning permission and Building Regulations approval are separate legal processes, and passing one says nothing about the other.
What the Drawings Actually Show
A planning drawing communicates appearance and footprint; a building regulation drawing communicates how the extension is actually built — foundation type and depth, wall construction and insulation, floor and roof construction, structural openings and their beams, drainage layout, means of escape where relevant, ventilation, and thermal calculations under Approved Document L. It has to be enough for a surveyor to check compliance and a contractor to build from without guessing.
This is where confusion — and cost — creeps in. A homeowner with planning permission, or building under permitted development with no planning drawings at all, still needs a separate building regulation submission before work starts. Neither process substitutes for the other.
Full Plans or Building Notice
There are two routes to approval. A full plans application submits complete drawings and structural information before work starts; the authority has up to five weeks (or two months by agreement) to approve or reject, with approval valid for three years. A building notice allows work to start with less advance documentation, checked as it proceeds.
Full plans is normally the better route for an extension — it confirms compliance before anyone is on site, which matters for new structural openings, work near a shared boundary, or a fixed-price build. A building notice can suit simple, low-risk alterations, but it shifts compliance risk onto the build itself, and isn’t available for larger schemes or work near a public sewer.
Where Structural Calculations Fit In
Most extensions involve at least one structural change: removing a wall, forming an opening, supporting a new floor, altering a roof. The drawings show what the finished construction looks like; calculations prove it will stand up, sized against actual span, loading and existing construction — not a generic table. Architectural and structural design need developing together here, not sequentially.
If beam layout, padstones and bearing points aren’t confirmed before the drawings are finalised, they either get reissued once calculations arrive, or the contractor resolves a query on site — slower and costlier than resolving it on paper. Coordinating both disciplines from one team, rather than handing drawings to a separate engineer afterwards, is why integrated architectural and structural services exist.
What Building Control Is Checking
Building control isn’t checking the extension against what was agreed at planning stage — that’s separate. A surveyor checks drawings and calculations against the current Approved Documents: structure, fire safety, moisture resistance, ventilation, drainage, thermal performance, access, among others. Part B was updated March 2025 and Part T November 2025, so the version in force at submission is worth confirming, not assumed. Approval of drawings isn’t sign-off that the finished building complies — site inspections and a completion certificate confirm that.
Where Party Wall Matters Overlap
Excavation near a shared boundary, or work to a party wall itself, brings in the Party Wall etc. Act 1996 as a third, separate process. A notice doesn’t grant permission to build — it’s a statutory step that can lead to an Award setting out how work is carried out. Where foundations sit close to a neighbouring structure, the drawings and the Party Wall position need looking at together: a foundation solution chosen for structural reasons can trigger, or avoid, the need for notice.
What You Receive, and What It Costs
A complete package normally includes existing and proposed plans, sections and elevations at construction detail, structural drawings and calculations for any altered structure, drainage layout, and the supporting information building control needs. Scope varies — a single-storey rear extension needs less than a scheme involving steelwork or basement works — and fees follow the same logic: complexity, structural content, the full plans/building notice choice, and how much existing survey information is already available before design work can start.
Programme follows the same factors. A straightforward extension with modest structural changes moves through drawings and calculations faster than one involving significant steelwork or a scheme close to a boundary where the Party Wall position also needs resolving. A full measured survey done early is usually the biggest single factor in how quickly the rest of the process runs.
Next Step
Once planning permission is in place, or the extension falls under permitted development, the next stage is Building Regulations — and the drawings and calculations are worth commissioning together, for the reasons above. AC Design Solution prepares both as one process across London, and can advise on the full plans versus building notice decision for a specific property.
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