Building under permitted development feels simpler than a planning application because there's no submission or waiting period, but that simplicity is exactly why it's worth double-checking before work starts. There's no council sign-off along the way to catch a mistake — the responsibility for getting the assessment right sits entirely with you or whoever advised you.
This is a working list of the things worth confirming before treating a proposal as permitted development.
Has a contractor's opinion been treated as a fact?
A builder saying 'that'll be fine under permitted development' is not the same as a confirmed assessment, however experienced they are. Ask specifically what limit the scheme has been checked against — depth, height, boundary proximity — and whether that check accounted for any previous extensions to the property. If the answer is vague, that's a sign to check independently or apply for a lawful development certificate.
This matters because the consequence of being wrong isn't a fine paid by the contractor — it's an unlawful structure that you, as the property owner, are responsible for regularising, potentially years later.
Have earlier extensions by a previous owner been accounted for?
Permitted development allowances are cumulative from a fixed reference point, not reset when a property changes hands. If a previous owner already built a single-storey rear extension, that volume counts against what you're now allowed to add, whether or not you knew about it when you bought the house.
Check the planning portal for the property's history and, ideally, ask your solicitor to confirm this was reviewed during the original purchase, since it's easy for this to go unchecked at conveyancing stage too.
Is the property in a conservation area, AONB, or under an Article 4 direction?
These designations reduce or remove permitted development rights that would otherwise apply, and none of them are obvious from simply looking at the house. Check with the relevant council — Ashford, Maidstone, Tunbridge Wells, Tonbridge & Malling or Rother, depending on the address — rather than assuming a similar-looking house nearby sets a precedent for yours.
An Article 4 direction can apply to a small, specific area within a wider borough, so a general awareness that 'this area is protected' isn't a substitute for checking the specific designation covering the actual plot.
Have height and boundary limits been measured precisely?
Permitted development limits are often measured to specific figures — a maximum eaves height, a set distance within which extension height is further restricted near a boundary. A design that's a small amount over one of these limits doesn't scrape through; it falls outside permitted development entirely and needs a planning application.
Ask for the actual measurements against the actual limit, not a general statement that the design 'should be fine' — the margin for error here is genuinely that tight.
Is there a plan for what happens at the point of sale?
Think ahead to when the property is eventually sold. A buyer's solicitor will often ask for evidence that any extension built without planning permission was lawful under permitted development at the time. Without a certificate or clear records, this can delay a sale or force a retrospective certificate application under time pressure.
Keeping dated photographs, measurements and a written note of which rule was relied on, from the point the work is planned rather than after it's built, avoids that scramble later.
Common questions
Can I apply for a certificate after the work is already built?+
Yes, this is called a certificate of lawfulness for existing development, but it relies on being able to prove when the work was carried out and that it complied with the rules at that time, which is harder to demonstrate retrospectively than to confirm in advance.
What's the risk if I get it wrong?+
The council can serve an enforcement notice requiring alteration or removal of unlawful work, though there are time limits after which action generally can't be taken. Either way, it's a risk and cost worth avoiding by checking properly beforehand.
Does a mortgage lender care about this?+
Yes — lenders and future buyers' solicitors commonly ask for evidence of lawful planning status, and an unresolved question here can complicate or delay a sale or remortgage.
Get your permitted development scheme checked before you build
We'll measure the actual proposal against the limits that apply to your property and flag anything that needs a closer look.