A basement flood risk assessment costs from £570 at Fixed Fee Plans. It assesses every way water could reach a new or extended basement, from a breach of the Thames defences to groundwater and the sewer backing up, and applies your council's own basement flood rules to the use you propose. We write it alongside our basement drawings, so the measures appear on the plans.
It is a separate product from the householder flood risk assessment because basements are judged differently. Several London councils ask for one whatever the flood zone.
Do I need a flood risk assessment for a basement?
In much of inner London, yes, even outside the flood zones. Kensington and Chelsea treats basements as highly vulnerable and requires an assessment for any basement development, regardless of the flood zone. Southwark lists basement extensions among the developments that must submit one. Hammersmith and Fulham requires one where its Surface Water Management Plan shows elevated groundwater. We check your council's rule before quoting.
What use can the basement have?
That depends on the flood zone and, near the Thames, the breach hazard. Richmond does not permit bedrooms at basement level in Flood Zone 3a areas of extreme, significant or moderate breach hazard, and any basement it allows must have internal access to a higher floor. Southwark does not support basements for sleeping accommodation at all and wants them used mainly for storage, servicing or parking. Wandsworth and Merton do not permit self-contained basement flats in their higher flood zones.
So the assessment starts by placing the house on the breach hazard and groundwater maps, then tells you plainly which uses are open. A self-contained basement flat in Flood Zone 3 needs a hydrologist's assessment, which we quote separately; often the answer is that the council will not accept it.
How is a basement protected from sewer flooding?
With a pumped device or non-return arrangement, and the assessment says which. Merton asks basements that discharge to the sewer to install suitable positively pumped devices against sewer flooding, and other councils set similar conditions. We note the device, the raised lightwell walls, flood doors and resilient finishes the site needs, and mark them on the basement plans and sections.
What else does a basement application need?
Usually a basement impact assessment by a chartered engineer, which covers ground movement, groundwater flow and neighbouring buildings; Southwark asks for one with every new or extended basement. It is a separate engineer's report. Our fee is from £570 for a desk assessment of one house; a large or complex basement is quoted in writing before we start.
Frequently asked questions
Does a basement in Flood Zone 1 need a flood risk assessment?
It can. Kensington and Chelsea requires one for any basement regardless of flood zone, and Hammersmith and Fulham where groundwater is elevated. Other councils follow the national flood zone rule. We check yours before quoting.
Can I have a bedroom in a basement near the Thames?
Often not. Richmond does not permit basement bedrooms in its higher breach hazard areas of Flood Zone 3a, and Southwark does not support basement sleeping accommodation. The assessment tells you what your council allows.
Is this the same as a basement impact assessment?
No. A basement impact assessment is an engineer's report on ground movement, groundwater and neighbours. The flood risk assessment covers flooding. Most London basement applications need both.
