How to Get Planning Permission in Tonbridge and Malling: An Architect’s Guide (2026)
03/07/2026
What Your Builder Won't Tell You: 8 Extension Mistakes an Architect Can Prevent
Good builders are worth their weight in gold, and this article is not against them. But the builder's job begins where the drawings end — and by then, the most expensive mistakes have usually already been made, silently, in decisions nobody flagged. These are the eight I see most often when picking up projects halfway, and every one is preventable on paper.
1. Pricing the project from one quote on thin drawings.
Without proper drawings, a quote is a guess; without competition, it's an expensive guess. The gap surfaces on site as extras, priced mid-project when you can't walk away. Prevention: full technical drawings, three tenders, written contract.
2. Solving floor area and creating a dark middle room.
The extension gleams; the room behind it dies. Daylight for the *old* space has to be designed — rooflights at the junction, borrowed light, a rethought plan. Prevention costs a drawing; the retrofit costs a roof.
3. Ignoring the planning designations until refusal.
Conservation area, Article 4, TPO on the sycamore, estate covenants — around Tonbridge these are everywhere, and each reshapes the design. Prevention is an hour of checking before the first sketch; cure is a lost season and, since 2025, a full second application fee.
4. Treating structure as the builder's problem.
'We'll sort the beam on site' is how bounce, cracking and £4,000 emergency steels happen. Prevention: engineering designed and coordinated with the architecture before tender — including where the loads land in the old house.
5. Forgetting the systems the new space depends on.
The bigger house outruns the old boiler; the new bathroom outruns the hot water cylinder; the drains run exactly where the footings want to be. Prevention: services and drainage designed at Stage 3, not discovered at week 6.
6. Glazing without a heat strategy.
South- and west-facing glass without shading design produces a room unusable on the days you most wanted it. Building regs now ask the overheating question; ask it of your design before they do.
7. Serving party wall notices when the scaffolder is booked.
Two months' statutory notice means the Act sets your start date, not your builder. Prevention: notices served at planning submission — and the neighbour told over coffee first.
8. Spending the contingency in your head.
The 10–15% contingency is for the ground, the unknowns in old fabric, and the genuinely unforeseeable — not for upgrading the kitchen in month two. Projects fail at the end for money spent at the beginning. Prevention: treat contingency as insurance until the roof is on.
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The pattern behind all eight
Every one of these mistakes is cheap to prevent and expensive to cure, and the prevention always happens in the same place: on paper, before tender. That — more than taste, more than flair — is what design fees actually buy: the mistakes you never got to meet.
Want your project checked against all eight?
TP Evans Architects reviews extension plans at any stage — including drawings you already have. Get in touch.
Planning a project in Tonbridge or the surrounding villages?
TP Evans Architects is a Tonbridge practice working across Kent. We design extensions and renovations and manage the whole planning process with Tonbridge & Malling Borough Council — from feasibility and pre-application advice through to approval and construction. Get in touch for an initial conversation about your project.
