If somebody tells you an unauthorised extension becomes safe after four years, they're working from a rule that no longer exists in England. It was replaced in April 2024, and a lot of advice online still hasn't caught up. This walkthrough of Fulham architectural design and the planning routes behind it covers the ground properly, because the change matters enormously if you've inherited work you can't find paperwork for.
We get this call regularly. Someone buys a Victorian terrace in SW6, the survey flags a rear addition with no permission on record, and the seller shrugs and says it's been there years.
Years might not be enough anymore.
What Actually Changed on 25 April 2024
Section 115 of the Levelling Up and Regeneration Act 2023 amended section 171B of the Town and Country Planning Act 1990. The words "four years" were deleted and replaced with "ten years" for England.
Before that date, unauthorised building work became immune from enforcement after four years from substantial completion. Change of use carried a ten year clock. Two different rules, which is where the confusion came from.
Now there's one. Every breach of planning control in England becomes immune after ten years, not four. Wales kept the four year period, which is another reason bad advice keeps circulating.
There is one important carve out. The ten year limit does not apply where the operational development, or the change of use to a single dwelling, was substantially completed before 25 April 2024. Work finished before that date keeps the old four year immunity.
So the date your extension was completed decides which rule governs it. That single fact is the whole assessment, and it's the first thing Fulham architects should establish before drawing anything.
Which Clock Applies to Your Fulham Property
The immunity period for a planning breach depends on the type of breach involved. If the building work was completed before 25 April 2024, the immunity period is four years, and the clock starts running from the date the operations were substantially completed. If the building work was completed on or after 25 April 2024, the immunity period increases to ten years, with the clock again starting from the date the operations were substantially completed. In the case of a change of use or a breach of a condition, the immunity period is also ten years, but here the clock starts from the date the breach occurred or the date the condition was first broken. Works carried out to a listed building, however, are treated differently there is no time limit at all, meaning such a breach never becomes immune, regardless of how much time has passed.
That last row deserves attention in a borough with as much period stock as Hammersmith and Fulham. Listed building breaches never become lawful through the passage of time. There is no waiting it out.
Demolition in a conservation area sits outside the immunity framework too, which matters given how much of the borough is designated.
Why Concealed Work Is Riskier Than People Assume
The legislation anticipated people hiding unauthorised work until the clock ran out. Sections 171BA to 171BC allow a council to apply for a planning enforcement order where a breach has been deliberately concealed.
If granted, the immunity period effectively restarts. So the calendar only protects work that was carried out openly.
That's worth knowing before anyone suggests keeping quiet and waiting. An honest breach gains time. A hidden one can lose the protection entirely, and it colours how the council treats everything else about the property.
"The question we ask first is never whether the work looks acceptable. It's when it was finished, and whether that can be evidenced. Two houses with identical unauthorised extensions can be in completely different legal positions purely because of a completion date." Eugene Kim, Extension Architecture
How a Lawful Development Certificate Fixes the Paperwork
Where the relevant period has passed, you don't just rely on it silently. You apply for a certificate of lawfulness under section 191 of the 1990 Act, confirming the existing development is lawful.
This is the document that unblocks a sale. Buyers' solicitors ask for it, lenders want it, and without it the unauthorised work sits on the title as an open question forever.
The evidence burden has grown with the rule change. Anyone now relying on the ten year period needs to demonstrate a full decade, which means photographs, invoices, council tax records, utility connections, aerial imagery, statutory declarations from neighbours.
Four years of evidence is achievable for most people. Ten years is a genuine research exercise, and it's why these applications now take longer to prepare than they used to.
The Other Route: Retrospective Permission
If the clock hasn't run, a certificate isn't available. What's left is a retrospective planning application, which is simply a normal application for work that already exists.
The council judges it on the same policies it would have applied beforehand. That means a rear dormer built badly in a conservation area is likely to fail, while a competently designed one may well pass.
We've handled over 1,800 planning approvals across London and Surrey, and the retrospective cases that succeed have one thing in common. Somebody assessed honestly whether the work as built could be defended on policy, and where it couldn't, proposed alterations rather than arguing.
Refusal on a retrospective application doesn't automatically mean demolition. It usually means a negotiation about modifications, and that conversation goes far better when it starts before an enforcement notice lands.
Loft conversions in Fulham typically take five to seven months from first design meeting to completion, and a well documented planning position from the outset is what keeps that programme intact.
