Guide · Extensions
Planning an extension in Kent: permission, building control, and the order to do it in.
Most of the money wasted on an extension is spent before anyone lifts a shovel — on drawings for a scheme that was never going to get permission, or on a quote that turns out not to include the steel. This is the sequence we wish everyone knew about before they rang a builder, and the six decisions that actually set the cost.
General guidance only. Planning and building regulations change, and what applies to your property depends on the property. Always confirm with your own local authority's planning and building control departments before committing to anything.
The order to do it in
Six stages. Doing them out of order is what costs money, because each one can invalidate the last.
Work out the planning route first
Before you commission anything, establish whether the extension you want falls under permitted development or needs a planning application. The rules are national and published — the Planning Portal sets out the current limits — but permitted development rights are restricted or removed on flats and maisonettes, on listed buildings, in conservation areas and Areas of Outstanding Natural Beauty, and where a previous owner has already used them up. Parts of the Tunbridge Wells and Tonbridge & Malling areas sit inside designated land, so this genuinely matters here.
If it is permitted development, consider certifying it
You can build under permitted development without telling anyone. You can also apply to the council for a Lawful Development Certificate, which confirms in writing that the work was permitted. It costs a fee and takes weeks rather than months — and it is the document a buyer’s solicitor will ask for in ten years’ time. Where a large single-storey rear extension relies on the prior-approval ("neighbour consultation") route, that notification has to be made before work starts, not after.
Drawings, then structure
An architect or architectural technologist produces the plans; a structural engineer produces the calculations and beam schedule for anything load-bearing. On a simple extension you may need very little of the first and a page of the second. On a job that removes most of a rear wall, you need both. Getting a builder to quote from a survey drawing alone is how quotes end up not comparable with each other.
Building regulations — a separate approval entirely
Planning permission decides whether you may build it. Building regulations decide whether what you build is sound: structure, foundations, damp proofing, insulation and thermal bridging, ventilation, fire escape from the rooms behind it, drainage, glazing and electrics. You apply either to the local authority’s building control service or to an approved inspector, and inspections happen at set stages — foundations before they are covered, drainage before backfilling, insulation before it is boarded over. The programme has to leave room for those visits.
The Party Wall etc. Act, if you are near a boundary
Excavating within three metres of a neighbour’s structure and below its foundation level, or cutting into a shared wall, brings the Party Wall Act into play. That means serving notice on the neighbour — normally two months before work starts on a party wall, one month for excavation — and if they dissent, appointing a surveyor. It is a procedure, not a fight, but it has statutory timescales and it is the single most common cause of an extension start date slipping.
Then, and only then, get quotes
Once the planning route is settled and you have drawings and structural calculations, every builder is pricing the same job and you can compare the numbers meaningfully. Ask each of them what is excluded, who is dealing with building control, and what happens to the price if something unexpected turns up. Our own quotes are free and carry no obligation, and we would rather talk you out of a job that is wrong than be the cheapest number on a bad plan.
What actually sets the cost
Six decisions, made early, that decide the number.
Square metres are the worst way to price an extension, because two extensions of identical size can differ enormously. These are the six things that make the difference.
01
How far out, and how high
Depth, height and roof form are the three dimensions that decide both whether you need planning permission and what the structure costs. A single-storey rear extension that stays inside permitted development limits is a very different job from one that needs an application — and an extension whose roof has to be tied into an existing one is a different job again from a flat-roofed box.
02
Where the drainage runs
If you are building over or within three metres of a public sewer you will need a build-over agreement from the water company, and that takes time to obtain. Find out where the drains run before you finalise a footprint — it is far cheaper to shift a wall on paper than to divert a sewer.
03
What is holding the back of the house up
Removing the rear wall to open into an extension almost always means a steel beam and new padstones, and sometimes means the existing foundations need underpinning locally. This is what a structural engineer is for, and it is money well spent — the beam schedule also tells you what the crane or gang has to lift on the day.
04
How much glass, and which way it faces
Glazing is usually the single biggest line in an extension quote after the shell. Bi-folds, sliders and rooflights all behave differently in terms of cost, thermal performance and how much of the wall they take out. A south or west elevation with a lot of glass and no shading will overheat — which building regulations now take a view on.
05
Whether the new floor level matches the old
An insulated floor build-up in a new extension is thicker than the floor in a 1930s house. If you want a level threshold between the two, the difference has to be designed in at the slab stage — not discovered when the screed goes down.
06
Matching the existing house
Brick, render, tile hanging and weatherboard all age. A new brick that matches a sample will not match a wall that has been weathering for ninety years, and you should know that before you choose. Sometimes the honest answer is to stop trying to match and make the new work deliberately different instead.
Thinking about an extension in Pembury, Tunbridge Wells or Tonbridge?
Ring Matt before you commission anything. He will tell you which planning route your job is likely to need, what it will genuinely involve, and whether it is worth doing at all. The visit and the written quote are free, with no obligation.