Planning's through. The client thinks the hard part is over. And someone in the office has just said the words "can you put the building regs package together" as though that were a small administrative task rather than the point where the drawings stop being pictures of a building and start being instructions for building one.
Here's the good news: the package is smaller than you fear and more repetitive than you'd like. A single-storey rear extension needs maybe six to eight sheets. Once you've done one properly, the next one is largely the same set with different dimensions. The bad news is that Building Control will read it in a very specific order, looking for very specific things, and the query letter you get back will almost always be about the same handful of omissions.
There are two ways into Building Control for domestic work, and nobody online seems to explain what they mean for your drawing board.
Full Plans. You submit a complete set, the checker assesses it, and you get a decision notice. Approval in advance. Everything below applies.
Building Notice. You give notice, you start work, and the inspector checks it on site as it goes. Technically you can get away with far less drawn information — a location plan and a description might be enough to lodge it.
Everyone new to this looks at Building Notice and thinks: brilliant, less drawing. Don't. A Building Notice gives you no approved set to point at when the inspector disagrees with the contractor on site, no document to price against, and nothing to hand a warranty provider or a future conveyancer. In practice we produce the same package either way — the difference is only whether it gets stamped before work starts or argued about during. The drawings aren't for Building Control's benefit. They're for yours.
The other early call: Local Authority Building Control or an Approved Inspector (now registered building control approvers). The scope of what they check is the same. The temperament isn't. Approved inspectors tend to be faster and more commercially minded; local authority teams tend to want more drawn on the page. Ask, before you set the sheet list, which one you're going to.
The Sheet List
For a typical single-storey extension, at A1 or A3:
Location plan, 1:1250. OS extract, site edged red. Boring, mandatory, forgotten surprisingly often.
Site/block plan, 1:200 or 1:500. This is where the drainage lives, and where boundary distances get proved.
Existing plans, elevations, sections, 1:50 or 1:100. Recycled from the planning set.
Proposed GA plans, 1:50. The workhorse.
Proposed elevations, 1:50 or 1:100.
Proposed sections, 1:50, with at least one bay at 1:20.
Construction details, 1:20 and 1:5.
Notes and specification sheet.
That's it. If you're doing a loft conversion or taking out a spine wall, add a structural layout coordinated with the engineer. Everything else is padding.
The scale jump from planning to building regs is the whole story. Planning drawings are usually 1:100. Building regs drawings are 1:50 and below, because at 1:100 you physically cannot draw a cavity closer, and a drawing that can't show one isn't a construction drawing.
Your GA Plans Are Not the Planning Plans With Better Hatching
The proposed plan at 1:50 has to carry a different payload. Working left to right across what a checker actually hunts for:
Wall build-ups tagged, not described. Every wall type gets a reference — W01, W02 — that ties to the notes sheet. Draw the cavity to its real width. A 350mm wall might be 102.5mm facing brick, 100mm cavity, 100mm block and 12.5mm plasterboard on dabs — and it should measure that on the drawing.
Structure indicated. Beams shown dashed, with references matching the engineer's scheme. You are not designing them. You are coordinating them, and the checker wants to see that the beam you've drawn and the beam the engineer has calculated are the same beam.
Drainage. New runs, gradients, inspection chambers, the connection point, and how you've dealt with any run that passes near or under the new foundation. Rainwater goods and where they discharge.
Ventilation and services. Background ventilators marked on each room, extract points in wet rooms, smoke and heat alarm positions with their interlink noted.
Thresholds and levels. Finished floor levels inside and out, on the plan as well as the section.
Section call-outs. Every cut you've drawn, marked where it's taken, with the sheet it lives on. We teach this as its own topic because a set where the details float around unreferenced is a set the contractor will ignore — and a checker who can't find your eaves detail will simply write and ask for it.
The Section Is Where the Package Is Won
One 1:50 section through the whole extension for setting out and heights. Then one bay at 1:20 that resolves the build-up from the bottom of the foundation to the top of the roof in a single vertical line.
That 1:20 is the most useful drawing in the set, and it's the one juniors skip. It has to answer, without ambiguity:
Foundation type and depth, cross-referenced to the engineer, with the note that final depth is agreed on site after trial holes.
Ground floor construction — slab or beam and block — and where the floor insulation sits relative to the wall insulation.
The DPC and DPM, and how they lap. This is the single most common thing to be drawn vaguely and then asked about.
The cavity: closed at the top, closed at openings, insulation continuous.
Floor-to-ceiling height, structural depths, and whether your beam actually fits in the zone you've drawn.
Roof build-up and whether it's warm or cold. Warm roof is best practice — insulation above the structure, structure kept warm, lower thermal bridging risk. Cold roofs give you a thinner build-up when you're fighting for headroom, and then they need ventilating and the vapour control layer is more easily punctured by anything screwed to the ceiling. Whichever you choose, the section must show it clearly enough that nobody has to guess.
One thing worth saying loudly: you cannot finish this drawing on your own. The slab depth comes from the structural engineer. The insulation thicknesses come from the energy assessor. The duct zones come from whoever's doing the mechanical side. You are the puppet master pulling those inputs into one coherent line — not the person inventing them. Drawing a foundation depth you've guessed at is how people end up in a claim.
Four Details Carry the Whole Set
For a masonry extension, the same details keep coming round:
Foundation and wall base — footing, slab, DPC-to-DPM lap, insulation continuity at the perimeter, external ground level.
Intermediate floor — joists, hangers bedded into the joint, strapping, how the floor zone meets the cavity. Only if you're going up.
Flat roof and upstand or parapet — deck, vapour control layer on the warm side, tapered or layered insulation with the thickest layer outermost, waterproofing, the edge condition.
Add a fifth for openings: a head, jamb and cill at 1:5, with the lintel, cavity tray with stop ends, weep holes, the cavity closer and the window position in the reveal. That one drawing prevents more site queries than any other.
Draw those four or five properly and you've covered the vast majority of what a residential masonry job throws at you. Everything else is a variation.
If you've never built a detail from scratch — and if all you've drawn at Part 2 is a 1:20 bay for a crit, you haven't — this is the gap to close before you send anything to a checker. Architectural Detailing Basics Vol 1 works through exactly these four junctions, in 2D and then modelled in 3D, so you can see how the layers actually meet rather than hoping they do.
Write the Performance, Draw the Geometry
Here's the split that saves you time: drawings carry geometry, notes carry performance. Don't try to draw a U-value.
Two ways to label. Either annotate each detail individually, or build an element key — one file, one consistent set of labels, copied onto every sheet. The key eliminates duplication, keeps your terminology identical across the set, speeds up labelling massively on anything larger than a rear extension, and doubles as an outline specification and a shopping list for the contractor. Duplication is the enemy. One key, every sheet.
A notes block on a real drawing reads something like this:
FOUNDATIONS: Mass concrete strip to Structural Engineer's drawing [ref]. Final depth to be agreed on site with Building Control following trial hole inspection.
DPC: Bitumen polymer DPC, minimum [X]mm above finished external ground level, lapped minimum [X]mm with the floor DPM and fully sealed.
CAVITY WALL: Facing brick / [X]mm cavity with partial-fill insulation / blockwork type and density to SE. Insulation thickness to achieve the U-value stated in the Energy Assessor's report [ref].
OPENINGS: Lintels to SE schedule. Proprietary insulated cavity closer to all reveals. Cavity tray with stop ends and weep holes over all openings.
VENTILATION: Background ventilators to habitable rooms and mechanical extract to wet rooms, rates as Approved Document F. Internal door undercuts as Part F guidance.
Notice what those bracketed placeholders are doing. They're not laziness — they're the discipline. Every number on a Building Regs drawing comes from a named source: the engineer's calculation, the assessor's report, the relevant Approved Document, or the manufacturer's certificate. Type one from memory and you own it. Reference it and the person who calculated it owns it. In our experience that habit is the difference between a set that survives a claim and one that doesn't.
Also worth naming: a straightforward extension can often be justified with U-value notes and an elemental approach, but some schemes — big glazed areas especially — need a proper SAP-based calculation from an energy assessor. Establish which you're on before you commit to a glazing ratio the scheme can't support.
The Three Omissions That Generate the Letter
Across the packages we've seen come back from checkers, the same three gaps do most of the damage.
Thresholds. The external door threshold is drawn as a vague blob, or the level change between the new floor and the patio isn't resolved. Approved Document M covers access into and within dwellings, and on extension work with new external doors the threshold is the thing a checker flicks straight to. Draw it at 1:5: the threshold section, the drainage channel, the DPC arrangement, the levels either side. It also happens to be the detail contractors get wrong most often, so you're solving two problems.
Cavity closers. Draw the closer at every reveal, head and cill. A cavity left open at an opening is a thermal bridge and a route for fire and smoke, and an unclosed cavity on a 1:5 detail is an instant query. Same with cavity barriers where the roof meets the wall.
Ventilation paths. Not just the ventilator on the plan — the whole path. Purge ventilation from an openable window, background ventilation, extract in wet rooms, transfer via door undercuts, and — critically — cross-ventilation of a cold roof void where a flat roof abuts an existing wall and the air has nowhere to go. That last one is a genuine construction problem dressed up as a drawing omission, and it's why we'd rather you detailed a warm roof.
Then Keep the Set Alive
Approval is not the end. The moment the contractor starts, things change — the trial hole comes up deeper, the steel goes up in size, the client moves a door.
Give every sheet a revision block from day one. Every change gets a letter, a date, a one-line description of what changed, and — if it matters — a cloud around the affected area. Reissue the whole set, not the single amended sheet, or you'll have three versions in circulation and the plasterer working off the earliest. Name files so a human can sort them: project, sheet number, title, revision. Issue as flattened PDF at the stated scale and check your line weights print. A beautifully coordinated set that arrives as a 90MB file of unflattened viewports is a set nobody will open.
The package isn't a hoop. It's the first time your drawings have to be right rather than persuasive, and the discipline it forces — everything referenced, nothing guessed, every junction resolved before anyone digs — is the discipline that makes you useful in practice. Take twice as long as you think you need. Everything in construction drawing does.
And if you want the regulations themselves rather than the deliverable, our guide on understanding Building Regulations covers what each Part is actually asking for.
