Certificate of Lawfulness (Existing Use): The 2026 Guide for HMOs and Converted Flats
Thousands of London landlords are operating property that is perfectly lawful — but couldn't prove it tomorrow if the council asked. It happens in two flavours: the HMO that has run for years without anything on paper, and the house split into flats long ago, with no planning permission anyone can find. With Article 4 directions spreading, enforcement powers strengthened, and lenders asking harder questions, the gap between being lawful and being able to evidence it has become a real commercial risk.
The instrument that closes it is the Lawful Development Certificate for an existing use — the "LDC (Existing)", formally a certificate under section 191 of the Town and Country Planning Act 1990.
What it is — and what it isn't
An LDC (Existing) is the council's formal, binding confirmation that the current use of your property is lawful for planning purposes. It is not a planning permission and is not judged like one: there is no assessment of amenity, parking or design. It is an evidential test — if you demonstrate, on the balance of probabilities, that the use is lawful, the council must issue the certificate. Once issued, it is conclusive: usable with enforcement officers, lenders, insurers, valuers and buyers.
What it isn't: an HMO licence, or Building Regulations sign-off. Those are separate regimes — a licence doesn't make a use lawful in planning terms, and a certificate says nothing about the standard of the works.
Case one: the established HMO
Under permitted development (GPDO Class L), a house can switch between a single dwelling (C3) and a small HMO of 3–6 sharers (C4) without planning permission — unless the borough has removed that right with an Article 4 direction. As mapped in our complete London HMO guide, 22 of 33 boroughs now have one, and the recent additions came fast: Sutton (February 2025), Lambeth's Streatham wards (August 2025), Ealing (November 2025), Hillingdon (December 2025), Merton (March 2026) — with Hammersmith & Fulham and Harrow in the pipeline.
The point landlords miss: an Article 4 direction is not retrospective. If your property was already in genuine C4 use before the direction took effect, that use began lawfully and remains lawful. But after designation the burden quietly shifts to you — if a neighbour complains or an enforcement officer calls, you must show the use predates the direction. Memories fade, tenants move on, agents close. An LDC (Existing) freezes the evidence while it is still fresh.
Two routes for HMOs:
The use began lawfully — you evidence that C4 use commenced while it was still permitted development. No waiting period applies: a use that began lawfully is lawful from day one. The application is about proving when it began and that it has continued.
The 10-year rule — a C4 use that began in breach (after an Article 4, without permission) becomes immune from enforcement after 10 years of continuous use. For HMO change of use this period has always been 10 years.
Case two: the unauthorised flat conversion
Splitting a house into self-contained flats has always required planning permission — it is a material change of use, with no permitted development route. London is full of conversions done decades ago without it: they let fine, until a sale, a re-mortgage, a lease extension or an enforcement query forces the question. Unauthorised flats are, in practice, unmortgageable and difficult to sell — which is why the LDC (Existing) is so often the document that unlocks a transaction.
Here the time limits changed recently, and the date matters enormously. The Levelling-up and Regeneration Act 2023 abolished the old "4-year rule" for breaches occurring from 25 April 2024, replacing it with a single 10-year tariff — but with a transitional saving:
Flats created before 25 April 2024 — the old 4-year rule still applies: each flat becomes lawful after 4 years' continuous use as a separate dwelling. Most pre-2024 conversions in London are therefore already immune — they just lack the paper to prove it.
Flats created on or after 25 April 2024 — the 10-year rule applies: no immunity until a decade of continuous use, with enforcement exposure for the whole period.
That is a genuine cliff edge. A conversion completed in March 2024 can be certificated from 2028; the identical conversion completed in May 2024 cannot be certificated until 2034. If you own a pre-2024 conversion that has passed the 4-year mark, there is no reason to leave it unevidenced.
One more nuance worth knowing: a self-contained unit with its own cooking and sanitary facilities is a flat, not an HMO, in planning terms — so "bedsit-style" buildings sometimes need careful analysis of which use is actually being certificated. Getting that description right is half the application.
The evidence that wins
The application stands or falls on documents. The strongest bundles combine several independent sources covering the whole claimed period:
Tenancy agreements — separate unrelated sharers (HMO) or separate households per flat
Council tax records — individual banding per flat is powerful evidence of subdivision
HMO licensing records — a licence doesn't confer planning status, but it evidences use
Utility accounts and metering — separate supplies per flat tell their own story
Electoral roll entries, bank/post records addressed to multiple occupiers or numbered flats
Statutory declarations from the landlord, agents and (ideally) former tenants
Dated photographs, works invoices, deposit registrations and agents' records
The standard is the balance of probabilities, and the burden sits with the applicant — the council doesn't have to disprove your case; you have to make it. Precision matters: the certificate describes a specific use ("use as a Class C4 HMO by up to 5 persons"; "use as 3 self-contained flats"), and you get exactly what you describe.
Risks and pitfalls
False or misleading information voids the certificate — it can be revoked, and obtaining one dishonestly is an offence.
Gaps in the timeline — voids, refurbishments, reversion to single-family use or a period of whole-house letting can break continuity and reset the clock.
Deliberate concealment of a breach can defeat the time limits altogether.
Describing the use wrongly — six claimed occupants with evidence for four, or "four flats" where one lacks its own facilities, weakens the whole application.
Waiting until enforcement (or a sale) starts — you can still apply, but you'll be assembling evidence under a deadline, against a sceptical audience.
What it costs
The fee for an LDC (Existing) is the same as the equivalent planning application (from 1 April 2026):
HMO change of use: £610 — £701 via the Planning Portal with its service charge.
Flat conversions: £610 per additional dwelling created — so a house converted to three flats is £1,830 in statutory fees, plus the Portal charge.
Professional preparation — the evidence audit, statutory declarations, the precise use description and the planning argument — is where these applications are won, and it is a fixed-fee service with us.
The bottom line
If your HMO predates your borough's Article 4 direction, or your flats predate April 2024 and have passed the 4-year mark, you are sitting on a lawful use you probably cannot yet prove — certificate it while the evidence is easy. If your use is still inside its qualifying period, take advice before doing anything that interrupts it. And if you're not sure which side of the line you're on, that's exactly the question a desktop review answers — contact us and we'll assess the evidence position before you spend anything.
Legal position verified 5 June 2026: s.191 Town and Country Planning Act 1990; Levelling-up and Regeneration Act 2023 enforcement provisions (in force for breaches from 25 April 2024, with transitional savings for earlier operational development and changes of use to a single dwellinghouse); fees per the Planning Portal schedule (1 April 2026). Borough Article 4 positions per our June 2026 verification. Take advice on your specific circumstances before applying.
