Mould is a symptom. It grows where a surface stays damp for long enough, so removing it without changing why the surface is damp buys a few months. Any quote worth accepting spends as much time on the cause as on the cleaning, and any quote that does not mention the cause is selling you a repeat visit.
Find the cause before any mould treatment
Mould in the corners of an external wall, behind a wardrobe, or around a window reveal is condensation on a cold surface. Mould in a defined patch that darkens after rain is penetrating damp with a source outside. Mould along the bottom of a ground floor wall may be rising damp or a bridged damp proof course. Mould in a bathroom with no working fan is a ventilation problem. Each has a different remedy and only one of them is solved by a chemical wash.
Removing it safely, and when to call a black mould specialist
Small areas on a hard surface can be cleaned by a householder with a proprietary fungicidal wash, ventilation, gloves and eye protection. Do not brush or dry scrape it, because that puts spores into the air, and do not paint over it. Where mould covers a large area, has got into plaster or plasterboard, or where someone in the house has a respiratory condition, that is the point to stop and get help. The NHS is clear that damp and mould affect health, and government guidance for rented housing says the same.
Professional mould removal cost: what the job includes
A survey that identifies the moisture source, containment so spores are not spread through the house, removal of contaminated material where plaster or board has been colonised, a fungicidal treatment, and then the work that stops it returning: ventilation, insulation of a cold bridge, or repair of the defect letting water in. Redecoration afterwards. A price that covers only the cleaning is cheaper because it is doing less, and it is worth knowing which you are being offered.
Landlords, tenants and the law
Government guidance is explicit that damp and mould in rented housing should not be dismissed as a lifestyle issue, and landlords have repairing obligations and duties around hazards. If you rent, report it in writing and keep the correspondence and photographs. If you let, treat a report as a survey trigger rather than an argument, because the building defects behind mould get more expensive the longer they run.
A mould survey, and what a mould surveyor should tell you
A report worth its fee names the cause and not just the growth. It should say where the moisture is coming from, whether that is a leak, rising or penetrating damp, or simply condensation from the way the property is heated and ventilated, and it should distinguish between them with readings rather than opinion. It should say which surfaces are affected and how deep, because growth on a painted surface and growth into plaster are different jobs. And it should say what to do about the cause, because anyone who reports mould and recommends only cleaning has described the symptom.
Questions people ask about mould on walls
Does bleach kill mould on walls?
It removes the staining on a hard surface and does not address what is feeding it. On porous plaster it often leaves the growth beneath. A proprietary fungicidal wash is better, and neither fixes the damp.
Why does mould keep coming back after cleaning?
Because the surface is still getting damp. Recurrence in the same place is the clearest evidence you have that the cause was never addressed.
Is black mould dangerous?
Damp and mould are associated with respiratory problems and are riskier for children, older people and anyone with asthma or a weakened immune system. That is the NHS position and it is why a large or persistent growth should not be lived with.
Can I paint over mould with anti mould paint?
Not as a fix. Anti mould coatings can help in a bathroom that is already ventilated and warm, and painting over an active growth on a cold damp wall hides it for a season.