A general arrangement plan says a wall is there. A detail drawing says what it is made of, where the insulation runs and what stops the cold bridge at the reveal. Six to twelve of them decide whether your extension gets built as designed or redesigned on site at your cost.
Building control wants the junctions where structure, insulation and the weather line all change at once. A typical London extension needs six to twelve details at 1:5 to 1:20. The most commonly omitted is the cavity tray over an abutment — invisible once built, and the usual cause of damp appearing eighteen months later.
| London householder work, 12 quarters to March 2026 | Figure |
|---|---|
| Householder decisions made | 77,680 |
| Householder approval rate | 83.0% |
| Decisions delegated to officers, not committee | 97.0% |
| Decided under an agreed extension of time | 34.5% |
| Withdrawn before any determination | 7.4% |
| Householder appeals allowed after refusal | 40.8% of 13,067 |
Source: MHCLG District Planning Application Statistics (PS1 and PS2), decisions made in the twelve quarters 2023 Q2 to 2026 Q1, retrieved 1 September 2026. Open Government Licence v3.0.
It shows one junction, drawn large enough that every material layer through it is separately legible. Take the eaves of a loft conversion. On a 1:100 section it is a corner. On a 1:5 detail it is eleven distinct decisions: the rafter depth, the insulation between and below the rafters, the ventilation path from soffit to ridge, the wall plate and its strapping, the cavity closer, the fascia and soffit line, the gutter position, the eaves membrane lapping into the gutter, the sarking, the breather membrane and the point at which the ceiling line meets the wall.
Every one of those has to be resolved before anyone builds it. The only question is whether it gets resolved at a desk, where changing it costs nothing, or on scaffolding at nine in the morning with a bricklayer waiting.
This is the difference between a drawing that describes a building and a drawing that instructs one. Our building regulations drawings carry the detail layer; the planning application drawings deliberately do not, because a planning officer is not assessing your insulation line.
There is no statutory list, which is why sets arrive incomplete. In practice a London building control body — whether that is the council or an approved inspector — wants to see the junctions where the thermal envelope, the structure and the weather line all change at once. Those are the places where buildings fail.
| Junction | What it has to resolve | Regulation in play |
|---|---|---|
| Foundation to ground floor | Damp-proof course level, insulation continuity under the slab, radon or gas membrane where required | Parts A and C |
| Eaves | Ventilation path, insulation depth, wall plate restraint, cold bridge at the wall head | Parts A, C and L |
| Window head, jamb and cill | Lintel bearing, cavity tray and stop ends, closer at the reveal, cill throating | Parts A, C and L |
| Flat roof upstand | Minimum 150 mm upstand, falls to outlet, membrane termination | Parts C and H |
| New build to existing wall | Movement joint or tie-in, cavity tray over the abutment, weathering of the flashing | Parts A and C |
| Party wall junction | Fire stopping at the cavity, acoustic separation, structural independence | Parts B and E |
The junction most often missing from a set submitted by a homeowner is the cavity tray over an abutment. It is invisible once built, it is cheap to install and catastrophic to omit, and it is the single most common cause of a damp patch appearing on the inside face of a new extension eighteen months after completion.
Scale is not a presentation choice. It decides what can physically be shown. A 50 mm insulation board drawn at 1:20 is two and a half millimetres wide on the page — thinner than the line weight used to draw it, so it disappears.
| Scale | Use it for | Smallest thing that stays legible |
|---|---|---|
| 1:5 | Cills, cavity trays, thresholds, reveals | Around 5 mm — a DPC, a sealant bead |
| 1:10 | Wall and roof build-ups, eaves, verges | Around 10 mm — a plasterboard layer |
| 1:20 | Storey-height sections, stair details | Around 20 mm — a batten |
| 1:50 | General arrangement sections | Around 50 mm — a stud |
If you are handed a set where every detail is at the same scale, it was produced by someone drawing to fill a sheet rather than to answer a question.
Building control rejection is not the main risk. The main risk is a set that passes building control and then cannot be built as drawn, because the details contradict each other or contradict the structural design.
The classic version is an insulation line that is continuous on the wall detail and stops dead on the eaves detail, drawn by two different people a fortnight apart. The building is now designed with a cold bridge nobody chose. Under Part L that is a thermal bridging calculation the builder has to resolve on site, and the resolution is usually to lose 45 mm of head height in a room that had none spare.
The second version is a detail that assumes a beam size the structural engineer has not confirmed. A 203 × 133 universal beam and a 254 × 146 occupy different amounts of ceiling. If the detail is drawn against the first and the calculations arrive specifying the second, every dimension below it moves. Our structural engineering drawings and the detail set are produced against the same model, so this class of contradiction does not arise.
Detail drawings are not sold separately here. They are part of the building regulations stage, and the building regulations stage is part of the Complete package.
| Package | Fixed fee | Includes details | Turnaround |
|---|---|---|---|
| Essentials — measured survey, existing and proposed plans | from £840 | No — planning stage only | 14–20 working days |
| Loft conversion package | from £1,225 | Yes | 14–20 working days |
| Mansard / roof rebuild package | from £1,575 | Yes | 18–24 working days |
| Complete — planning plus building regulations | from £1,750 | Yes | 20–28 working days |
For context on what the drawings are buying you: across the 33 London boroughs, 77,680 householder decisions were made in the twelve quarters to March 2026, and 83.0% were approved. Of all 163,144 decisions in that window, 97.0% were delegated to officers rather than going to committee, and 34.5% were taken under an agreed extension of time rather than inside the statutory period.
Source: MHCLG District Planning Application Statistics (PS1 and PS2), decisions made in the twelve quarters 2023 Q2 to 2026 Q1, retrieved 1 September 2026. Open Government Licence v3.0.
That last figure is the one worth acting on. An extension of time is agreed, not imposed — a case officer asks for one when the submission is nearly right but short of something. A complete set of details at the building regulations stage does not stop a planning extension of time, but it removes the equivalent delay downstream, where an incomplete set means the build stops rather than the paperwork.
A good detail is partly a set of references, and knowing what to leave out is as much of the skill as knowing what to draw. Nobody redraws a cavity tray, a proprietary lintel or a fixing pattern that a manufacturer has already tested and published; the detail names the product and the document, and the specification carries the rest. Redrawing it invents a component that has no test evidence behind it, and a plans checker will treat it as exactly that.
So a detail typically references a manufacturer’s installation instruction, a product certificate, the structural engineer’s member sizes, and where relevant a published standard construction detail for the junction. What it must still draw itself is the part nobody else can know: how those components meet this building, at this junction, with the existing fabric that is actually there. That is the drawing you are paying for.
A large-scale drawing of one junction in a building, normally at 1:5, 1:10 or 1:20, showing every material layer through that junction and how they meet. A general arrangement plan says a wall is there; the detail says what the wall is made of, where the insulation runs, how the damp-proof course laps the membrane and what stops the cold bridge at the reveal.
For a typical London extension or loft conversion, between six and twelve. The set almost always includes the foundation and ground floor junction, the eaves, the verge or parapet, a window head, jamb and cill, the ridge, and the junction between the new structure and the existing wall. More are needed where the roof is flat, where a party wall is involved or where the build-up changes partway along.
1:5 for anything where individual material thicknesses have to be legible, such as a window cill or a cavity tray. 1:10 for most wall and roof build-ups. 1:20 for sections through a whole storey where the point is the relationship between elements rather than the exact make-up of each one.
No. Planning is judged on what the building looks like and how it affects neighbours, so the planning set is plans, elevations and sections. Details belong to the building regulations stage, which is a separate application to building control and is where the construction is actually approved.
Not safely, and most will not price accurately from them. Planning drawings carry no build-up information, so every junction becomes a decision made on site by whoever is standing there. That is where cost overruns and building control rejections come from.
Our detail drawings are part of the Complete package at a fixed £1,750, which covers both the planning and the building regulations set. We do not bill hourly, so a junction that needs redrawing three times costs the same as one that does not.
Site plans are included in every drawing package we produce, on a licensed Ordnance Survey base with the ownership edging done properly.
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