Below ground, water is under pressure and it will find any weakness, so British practice has moved away from trying to seal a cellar and towards managing the water that gets in. Understanding the three approaches is what lets you read a basement quote, because the word tanking is now used loosely for all of them.
Basement tanking systems: the three types, in the trade's language
Type A is barrier protection: a tanking slurry or membrane applied to the structure to hold water out. Type B is structurally integral protection, where the concrete structure itself is designed to be watertight. Type C is drained protection: a cavity drain membrane on the inside face that lets water run down behind it to a perimeter channel and a sump, from where a pump lifts it away. Most domestic cellar conversions in Britain now use Type C, often combined with another type.
Why drained systems won
A tanked wall has to be perfect, and it has to stay perfect while the building moves, the substrate carries salts and someone later drills a hole for a shelf. Water under pressure finds the one defect. A drained system assumes water will get in and gives it a route out, which is far more forgiving. The trade off is that it depends on a pump and on maintenance: the channels need to be rodded and the sump and pump serviced, and a serious installation has a second pump and a battery backup because a power cut during a storm is exactly when it is needed.
Where waterproof tanking still makes sense
Tanking a wall is still the right answer in plenty of smaller situations: a retaining wall in a garden room, a partly below ground utility space, or a cellar where the water table is low and the aim is to stop damp rather than resist a head of water. It is applied to a sound prepared substrate, usually after removing the existing plaster back to masonry, and it needs a proper detail where it meets the floor. External waterproofing, applied to the outside of the wall, is the most robust of all and usually impossible on an existing house without excavating around it.
Tanking a basement cost: what moves the price
Area of wall and floor, how much of the existing finish has to come off, whether the floor is being included and to what level, whether a sump and pump are needed and how many, how the discharge reaches a drain, and the excavation if anything external is being done. Access matters enormously, because everything comes out through the house. A quote that does not say which type of protection is being installed, and whether a pump is in the price, is not a quote you can compare.
Guarantees, and what they are worth
A waterproofing guarantee is only as good as the company standing behind it and the maintenance conditions attached. Ask whether it is insurance backed, what it requires of you in servicing, and what is excluded. Ask also whether a waterproofing design specialist was involved in the design as opposed to a contractor quoting a product. Where the cellar is being made habitable, building regulations apply as well, including ventilation, escape and ceiling heights.
Questions people ask about basement waterproofing systems
What is the difference between tanking and a cavity drain membrane?
Tanking tries to hold water out with a barrier. A cavity drain membrane lets it in behind the sheet and drains it to a sump. The second is more forgiving and needs a pump and maintenance.
Does a tanked basement need a pump?
A true barrier system does not, in principle. A drained system does, and most domestic basement conversions in Britain use a drained system, so in practice a pump is usual.
Can I do cellar waterproofing myself?
Slurry products are sold retail and a small dry cellar can be improved that way. Anything involving a real head of water, or a space you intend to live in, wants a designed system and a specialist, because a failure means redoing the finished room.
Does waterproofing a cellar need building regulations approval?
Waterproofing alone often does not. Converting the cellar into habitable space does, covering ventilation, escape, and ceiling height among others.