Can I prove my extension is lawful after several years in London?

Four years or ten: which clock is your extension on?

For building work in England, section 171B now says no enforcement action may be taken after ten years beginning with the date on which the operations were substantially completed. Before the Levelling-up and Regeneration Act 2023 changed it, the figure for building work was four years.

The change did not reach back. Regulation 5 of S.I. 2024/452 says the amendments do not apply where the operations were substantially completed before the day the new section came into force, and the government's enforcement guidance gives that date as 25 April 2024. So, as a rough guide: a rear extension finished in spring 2020 had passed four years by spring 2024 and may qualify now; one finished in the summer of 2024 is on the ten-year clock and cannot qualify on time alone until 2034.

Two cautions. First, "substantially completed" is a question of fact, and a council may argue that an extension still missing its roof finish or windows was not complete when you say it was. Second, the same guidance records that where a breach is deliberately concealed, the courts have held the time limits do not engage until the breach is discovered. Loft conversions, outbuildings used as homes and houses split into flats each raise their own questions under section 171B, so treat the dates above as a starting point for advice, not the answer.

The evidence London councils look for

The burden sits with you. The guidance says the applicant is responsible for providing sufficient information, and that where the council has no evidence of its own to contradict yours, your evidence can justify a certificate if it is sufficiently precise and unambiguous to do so on the balance of probability. A few precise, dated records usually do more than a box of loosely related ones.

Councils publish what they accept. Southwark's checklist, for example, asks for at least 2 different types of evidence and lists building control certificates, rent documents, council tax or valuation letters that identify the unit, photographs, Google Street View screenshots and invoices for the construction work. Other boroughs word it differently, so we read your own council's list before we build the bundle.

Statutory declarations are the usual way to put personal knowledge in front of the council: the owner at the time, a neighbour who watched the work, the builder. Southwark says important statements from owners or applicants should be sworn in the proper form. We draft them from what each person tells us; each person then makes (declares) theirs before a solicitor, commissioner for oaths or another person authorised to take it, who charges separately. Council tax records help most where the work created separate living space, such as a flat, because they show the unit was treated as separate from a known date.

Why the drawings of the existing building matter

Section 191 requires the certificate to describe the use, operations or other matter in question. That description is only as good as the information behind it. An extension recorded as "rear extension" settles little; one shown on scaled plans and elevations, with its depth, height and footprint, settles exactly what is lawful and leaves no room for argument later about a bay or a parapet added afterwards.

We measure the house on a survey visit and draw the existing building as it stands. If you already hold plans from a past sale or an earlier application, check them before you rely on them: our guide on what to do if your drawings don't match your house explains why inherited drawings so often miss chimney breasts, outriggers and the true line of an extension. The drawings for a certificate are existing drawings only; our note on existing and proposed plans explains the difference.

How we prepare a certificate application

Most of the work happens before anything reaches the council. We start with the dates, because if the time period has not run, no amount of evidence will help, and we would rather tell you that on day one.

The steps below are the order we work in. You stay in control of the evidence: we never write a history for anyone, and we flag any gap rather than smooth it over, because the guidance allows a certificate to be revoked if a statement or document used was false.

What it costs, and what it does not cover

Our fee is £950 in total, including the survey visit. The council's own fee is separate and is paid to the council; for an existing operation it is the same as the fee for the matching planning application, which for alterations to a single house is the householder figure in the chart below. The person who takes the statutory declarations charges separately too.

If the extension has not been built yet, you want the other kind of certificate, for proposed work; what an LDC costs for a planned project covers that route and its lower council fee.

A certificate deals with planning only. It says nothing about whether the structure, insulation, drainage or fire escape met building regulations. If a buyer's surveyor raises those, the route is building regulations regularisation, which is a separate application to building control.

If the evidence is thin or the council refuses

A refusal is not the end of the matter. The guidance says a refusal is not necessarily conclusive that something is not lawful, and that it does not stop another application later if more information can be produced. Rights of appeal exist in some circumstances, and a planning solicitor can tell you whether one suits your case.

Before you apply, weigh one practical point: an application puts the history of the extension in front of the council. If the dates are borderline, or the evidence is mostly memory, it can be wiser to gather more first, or to consider a planning application instead. We tell you which way we think the evidence leans before you pay the council fee, and our certificate of lawful existing use service sets out exactly what the £950 covers.