1.The roof drawn right up to the bottom of the windows, with no gap

On the set we reviewed, the proposed extension’s roof was drawn running straight up to the underside of the window sills above it. No gap at all.

That gap matters. Where a new roof meets an existing wall beneath a window, you need lead flashing to weatherproof the junction and stop water tracking back into the building. If the roof is drawn hard up to the sill, there is physically nowhere for that detail to go. In practice it means the roof has to drop — which changes the pitch, and sometimes the head height inside — or water finds its way in.

A planning officer will not flag this. It does not affect how the extension looks from the street or how it impacts a neighbour, so it sails through planning untouched. It only becomes a problem when someone has to build it.

How to check yours:

You do not need to overlay anything or be technical about it. Just look at your proposed elevation drawing and find where the new roof meets the wall below a window. Is there a visible gap between the top of the roof and the bottom of the window? If the roof is drawn touching the sill, that is a problem to solve now.

2. A wall on the boundary with no allowance for the roof and gutters overhanging it

The drawings placed the side wall of the extension right on the boundary line with the neighbour. Even if you can build the wall itself within your land, there is a second issue people miss: the roof overhang and gutters project beyond the face of the wall. So even with the wall exactly on the line, the eaves and guttering would sit over the neighbour’s land.

That is not a technicality. You do not have the right to oversail a neighbour’s property without agreement, and it tends to surface as a dispute once it is built, not before. It usually means the whole extension has to be pulled in from the boundary to make room for the overhang, which shrinks the room you were trying to create.

How to check yours:

Look at where your extension meets any boundary and ask, “once you account for the roof overhang and the gutters, does anything project over the boundary line?” The wall position on its own is not the whole picture.

3. A roofline drawn so tight to the doors there is no room for the beam above them

The client wanted large bifold doors across the back — a very common request. You will not usually see a steel beam drawn on planning drawings, and you will not have structural calculations at this stage. But you do not need them to spot this one: the roof and ceiling line was drawn so tight to the top of the bifold doors that there was nowhere left to fit the steel beam and the roof structure that have to sit above those doors.

Think about it practically. A large opening like that needs a beam over it to carry the load, and the roof build-up sits above that beam. If the drawing runs the roof straight down to the top of the doors, where does the beam go? It is a spatial impossibility you can read straight off the drawing — and it is exactly the kind of thing that gets discovered on site, usually after the doors have been ordered to a height that no longer fits.

How to check yours:

If your design has bifolds, sliders or large glazed openings, look at the space between the top of the doors and the roofline above them. Ask, “is there enough height here for the structural beam and the roof build-up?” If it looks tight on the drawing, it is worse in reality.

4. A roof pitch too shallow and rooflights fitted below 15 degrees

The extension was drawn with a very shallow pitched roof. It looks fine on a drawing, but in practice it was too low for a standard pitched tiled roof to shed water reliably. Below a certain pitch, most roof tiles simply are not rated to keep water out, and you risk leaks.

The version of this we see constantly is Velux or rooflight windows installed in a pitched roof below 15 degrees. Most standard rooflights are rated for a minimum pitch of around 15 degrees — put one in a shallower roof than that and the manufacturer will not guarantee it stays watertight. It is an easy thing to draw and an expensive thing to leak.

Where a pitch genuinely needs to be low, the honest answer is often that it should not be a pitched roof at all. A flat roof with roof lanterns or skylights will perform better, look intentional, and let more light in. But that is a deliberate design decision, not something to fudge because the pitch will not work.

How to check yours:

If your extension has a low-angled pitched roof, check the pitch angle marked on the drawing. If it is below 15 degrees and there are rooflights shown in it, ask whether those specific windows are rated for that pitch. If the answer is not a clear yes, a flat roof with rooflights is probably the better solution.

5. Missing detail where new work meets an old structure

Part of the project involved building over an existing garage. The drawings showed the new structure — but not how it actually tied into and was carried by the existing garage walls below, which are very often not built to take an additional storey.

Again, this is not something planning cares about. But it is the difference between a project that has been thought through and one where a critical question — what is actually holding this up? — has been left as “to be resolved later,” which on site becomes an expensive surprise.

How to check yours:

If any part of your project builds on top of an existing structure (a garage, a single-storey element, an old outrigger), ask how the new load is carried down into something capable of taking it. “It’ll be fine” is not an answer.

Why this keeps happening

None of this means the firm who drew your plans was careless. Planning drawings and building regulations drawings are genuinely different jobs, answering different questions. Planning asks is this acceptable? Building regs asks does this work? A firm that only does the first one has no reason to check the second, and often does not have the on-site experience to spot it anyway.

The problem is that homeowners are rarely told this. You get an approval letter from the council, which feels like the hard part is done, and nobody mentions that the drawings you are holding have not yet been tested against how the thing actually goes together. That is why the errors cluster around the joins — roof to window, wall to boundary, roof to doors, new structure to old.

If you have got planning and any of the five warning signs above ring true, the fix is not necessarily to start again. It usually means a proper re-survey of the existing property, measuring what is actually there rather than what was assumed, followed by a set of building regulations drawings that resolve the clashes properly. Done at this stage, that is far cheaper than discovering the problems with a digger already on site.

Frequently asked questions

Does planning permission mean my drawings can be built?

No. Planning permission confirms the council is satisfied with the appearance of your extension and its impact on neighbours. It does not confirm the design is structurally sound, weathertight, or buildable. Those questions are answered at the building regulations stage, which is a separate set of drawings.

How do I know if my planning drawings are wrong?

The most common warning signs are a roof drawn right up to the bottom of a window with no gap for lead flashing, walls on the boundary with no allowance for the roof and gutters overhanging, a roofline drawn so tight to large doors there is no room for the beam above them, roof pitches too shallow for the tiles or rooflights specified (particularly rooflights below 15 degrees), and missing detail where new work is carried by an old structure.

My designer only did planning drawings. What happens next?

You will need building regulations drawings before you can build, and normally structural calculations alongside them. This is the stage where the design gets tested against how it will actually be constructed, and where any issues in the planning drawings should be picked up and resolved.

Is it expensive to fix problems in the drawings?

Fixing them at the drawing stage — a re-survey and a properly coordinated set of building regs drawings — is far cheaper than fixing them mid-build, where correcting a single boundary or roof error can run into thousands of pounds plus delays. Catching it early is almost always a fraction of the cost of catching it late.

Got planning permission but no building regulations drawings, and not sure your design will work?

Fluent Architectural Design Services provides building regulations drawings, structural coordination and pre-construction support across Surrey, South West and West London. Book a feasibility assessment to have your approved drawings reviewed properly before you build.