London has an enormous stock of solid walled Victorian and Georgian terraces, raised pavements, basements and flat conversions, and every one of those features produces damp that looks like rising damp on the ground floor. It is also a market with a very large number of firms offering free surveys and chemical courses. Both things being true at once is why an independent reading is worth more here than almost anywhere else in the country.
Raised pavements and lightwells
Pavement levels in London have risen repeatedly over a century and a half of resurfacing, and in many streets the external level now sits above the original slate damp course. A lightwell that drains slowly, or a coal vault whose grating has been sealed, does the same thing on the front elevation. All of these bridge a course that is otherwise intact, and none of them is corrected by injecting a new one above the old.
Flat conversions and where the water comes from
In a converted house the leak above is frequently in somebody else's flat, and the staining appears at the bottom of a wall two floors down after tracking through the structure. Where a party wall or a soil stack is involved the diagnosis needs access that a single visit does not get, and a survey confined to one flat can only report what it could see.
Basements, and the pressure question
London basement and lower ground floor rooms are commonly below the level of surrounding ground and, in parts of the city, close to a high water table. Below ground the choice between tanking and a drained cavity system is a judgement about water pressure rather than about moisture, and getting it wrong is expensive to undo. It should be argued in the survey, not assumed.
Questions people ask about rising damp London
Why do so many London houses get quoted for a damp course?
Because the visible symptom is common and the free survey model puts a contractor in front of it early. That is not evidence of bad faith, but it does mean the first opinion you receive is rarely an independent one, and a second reading is worth its fee.
Does a leasehold flat change what I should do?
Often. Where the cause is in common parts, the external wall or another flat, the freeholder or managing agent has the repairing obligation, and a survey that identifies the source is what you need before anybody starts arguing about who pays.
Is a basement damp problem always a waterproofing job?
No. A blocked lightwell gulley, a cracked rainwater pipe below ground or a defective drain will soak a basement wall exactly like groundwater. Drainage is cheaper to test and cheaper to fix, and a survey should exclude it first.