Check before the first drop of water.

The law, in two sentences

Under the Planning (Listed Buildings and Conservation Areas) Act 1990, section 7, you need listed building consent for any works that would affect a listed building's character as a building of special interest, whether the works are harmful or beneficial. Section 9 makes it a criminal offence to do such works without consent: on summary conviction up to £20,000 or six months, on indictment an unlimited fine or two years. The defences are narrow (urgent safety works, kept to the minimum) and routine cleaning or repointing does not fit them.

What that means for cleaning and repointing

• Minor like-for-like repair, a few joints repointed in matching lime, a cracked sill made good, is generally treated as maintenance and not as needing consent. The Act says nothing about repairs, which is why most conservation officers do not ask for an application for small ones.
• Substantial repointing, or a change of mortar or joint finish, usually needs consent, because it changes the character of the elevation.
• External cleaning of a listed building can need consent. In practice London councils grant it with conditions: test patches first, the method agreed in writing, and a result that does not look "as new".
• Painting, or stripping paint from, a listed front can need consent too.

Conservation areas

A conservation area is not a listing. Cleaning and repointing an unlisted house in one does not normally need consent by itself. But the council's area appraisal may protect features such as painted or unpainted fronts, and the officer will take an interest if the method could harm the street. Two London examples: Westminster has designated more than fifty conservation areas covering around three-quarters of the city; the Royal Borough of Kensington and Chelsea has 38 covering about 70 per cent of the borough. In those boroughs the honest assumption is that your building is in one.

What a firm should give you

1. A written method statement naming the cleaning system, temperature and pressure at the nozzle, chemicals (if any), and how run-off is contained.
2. Trial patch locations agreed with you and, where consent applies, with the officer.
3. A mortar specification for any repointing: lime type, sand, ratio, joint profile.
4. Photographs before, after the trial patch, and after completion, which the council may ask to see.

This page explains the rules in plain words. It is not legal advice and your borough's conservation officer has the final say on your building.