Basement tanking costs, and why three quotes never compare
Every published price for this job is quoted per square metre, and not one page says which square metre it means. What the two systems actually are, what each one cannot do, and the running cost nobody prints.
By The Damp GuyLast updated
The short version
Published rates run £40 to £100 per square metre for cementitious tanking and £80 to £150 per square metre for a cavity drain system. Those numbers look comparable and are not. Not one page publishing them states whether the square metre is wall area, floor area or both, and one firm prices the same job at £1,000 to £1,750 per square metre of floor area. On the same cellar that is a ten-fold difference caused by a unit, not by a price. Two sources publish the identical £80 to £150 rate, one saying the sump and pump are inside it, the other adding £1,500 to £2,500 for them. Before you compare anything, make each quote say what it measured and what is in the number.
Two different products, both sold as tanking
They fail in opposite ways, and that decides more about the cost of owning one than the installation price does.
The Property Care Association sets out three approaches. Type A is a structure "with no integral protection against water penetration" that "relies totally on a waterproofing membrane to keep water out". Type B is built with a water-resistant shell, usually reinforced concrete. Type C relies on a drained cavity that collects water and directs it to drainage or a sump.
In an existing house you are almost always choosing between the first and the third, and they are not variations on a theme. One holds water out. The other lets it in on purpose and pumps it away. The PCA states plainly that cavity drainage systems cannot withstand any hydrostatic pressure and are "only designed to control and manage MINOR leakage and seepage into a structure". If the drainage capacity is exceeded, "this may result in dampness or flooding".
The barrier system has the opposite profile. It can hold back a significant head of water and needs little or no ongoing maintenance, with no moving parts and no electricity. What it does instead is crack: "all cementitious systems are prone to shrinkage, so cracking and de-bonding is something that will invariably occur to some degree". And it cannot go on just any wall. Non-structural floors and half-brick walls "are not suited to the application of a cementitious system due to their inability to withstand bending stress".
| Type A, barrier (tanking) | Type C, drained cavity | |
|---|---|---|
| What it does with water | Holds it out. Relies totally on the membrane. | Lets it in behind a studded sheet and pumps it away. |
| Water pressure | Can hold back a significant head of water. | Cannot withstand any hydrostatic pressure. |
| What it is designed for | Keeping water out of the structure. | Controlling and managing minor leakage and seepage only. |
| How it fails | Cracking and de-bonding, which will invariably occur to some degree. | At joints and seams, or when drainage capacity is exceeded. |
| Depends on electricity | No. No moving parts. | Yes. The system will fail if the power supply breaks down. |
| Ongoing maintenance | Little or none. | Serviced at least annually, and a missed service voids the guarantee. |
| Finding the fault later | Renders and plasters must be removed before anyone can see where the break is. | Service points stay accessible, if the dry lining was not built out over them. |
| Where it cannot go | Non-structural floors and half-brick walls. | Where ingress exceeds what the drainage can carry. |
Every cell above is the Property Care Association's own published position, from its June 2022 Code of Practice for waterproofing existing below-ground structures and its June 2022 Type C best practice guidance. The PCA is a trade body whose members sell this work, which is worth knowing and does not make its technical guidance less useful. It is the only party in this market publishing what its members' own systems cannot do.
Why the published prices do not compare
Three pages, three rates, and no page in the entire ranking set defines its own unit.
Read the cost guides for this job and you will come away with a range that looks settled. It is not settled, and the problem is not that the sources disagree about money. It is that they are not measuring the same thing and none of them says so.
Three publishers price per square metre without ever stating whether that is wall area, floor area, or wall and floor combined. A fourth prices basement conversion at £1,000 to £1,750 per square metre of floor area, and also does not flag that its unit is different from everyone else's. Put those side by side on one small cellar and the answers are an order of magnitude apart, entirely because of a definition.
The second trap is smaller and sharper. Two sources publish the identical £80 to £150 per square metre for cavity drainage. One states that figure includes the sump and pump. The other lists the same rate and then adds £1,500 to £2,500 for the sump and pump as a separate line. Same headline number, and one of them is up to £2,500 more expensive. Neither page mentions the other convention exists.
| Published figure | Unit as published | Source and date |
|---|---|---|
| £50 to £100 | Per m2, cementitious tanking, supply and apply. Area not defined. | MPS Concrete, no date attached |
| £40 to £80 | Per m2, cementitious slurry. Area not defined. | Sussex Damp Experts, table labelled 2024 |
| £40 to £100 | Per m2, tanking slurry. Area not defined. | Alldamp, 14 May 2025 |
| £80 to £150 | Per m2, cavity drain membrane, stated to include the sump and pump. | MPS Concrete, no date attached |
| £80 to £150 | Per m2, cavity membrane, with the sump and pump priced separately. | Sussex Damp Experts, 2024 |
| £1,500 to £2,500 | Per unit, sump and pump, added to the rate above. | Sussex Damp Experts, 2024 |
| £1,000 to £1,750 | Per m2 of FLOOR area, basement conversion. | Trace Basement Systems, no date attached |
| £750 to £3,000 | Fee, waterproofing design specialist. | MPS Concrete, no date attached |
| approx £9.08 | Per m2, materials only, two coats of slurry. | Permagard, no date attached |
Nine published figures, four sources, and no two of them measuring the same thing. Every one is reproduced as its publisher states it, with no averaging: an average across sources that define their units differently would be a number that describes nothing. Five of these publishers sell either the work or the materials. The two directories that rank at the top of this search both refuse automated access, so nothing is quoted from either of them here.
The lines that arrive after the tanking quote
A quote for the waterproofing is not a quote for the job, and one page in six says so.
Only one publisher in this market states what its price excludes. It names insulation, plasterboard and skim, plumbing and electrical installation, structural alterations, doors, windows and joinery. The other five state no exclusions at all, which does not mean their prices include those things.
What that gap costs shows up on a forum rather than a cost guide. A homeowner with a tanking-only quote for a room of 3.3 by 3.3 metres then had to separately source the epoxy floor removal, breaking out the concrete channel, building the sump chamber, core drilling for the discharge, screeding over the membrane, the studwork, plasterboard, skim and rigid insulation, and the electrics. His own estimate for that lot, which he labelled a guess, was £6,000 to £10,000. The tanking was quoted separately and was not the biggest number.
The electrical supply is the part almost nobody mentions. A drained system runs on pumps, and the PCA's guidance on groundwater pumping stations is specific about what that means. Two pumps must always be used as a minimum, and each must independently meet the design duty rather than being sized on a duty-assist basis. Battery backup "should be included", able to power either pump for a 24-hour period at up to three activations per hour, fed by two independent spurs. A high-level alarm should always be fitted, giving an audible signal when the duty pump has failed, and it "should not have the option to extinguish unless manually muted". Where the backup is left out, the risk "should be fully understood and formally agreed by all parties", which tells you leaving it out is common enough to need a formal carve-out.
Then there is the servicing, and this is the number nobody in the market publishes at all. The PCA requires the channels and sumps to be cleared and tested immediately after installation, inspected again within three months, and serviced at least annually thereafter, sometimes considerably more often. Most contractors do not service their own systems, so this usually means a separate contract with a pump service engineer, and any alteration of the electrical circuits has to be done by a registered electrician. I could find no neutral published figure for what a UK annual service costs. That is not an oversight in the research. It is not published anywhere I could reach, on a cost that recurs every year for the life of the system.
The consequence of skipping it is written down plainly, and it is the sentence to read twice: any system failure resulting from a deviation from the planned maintenance programme "will not be the responsibility of the installer and will not be covered by any guarantee that has been issued".
What you are entitled to be handed, and one question that tests it
The trade body sets out what a waterproofing report must contain. Almost no homeowner knows it exists.
The PCA's code of practice lists what a waterproofing report should state, and the first item is the one most quotes skip: your required use of the basement, and therefore the design grade required. It also expects the existing construction, the ground type and permeability where known, the defects needing rectification, the proposed method, the suitability of the existing structure, condensation and ventilation, and a detailed specification of the chosen method.
The grade matters because it is chosen from what you intend to do with the space, not from what is convenient to install. The PCA's position is that habitable accommodation "should be designed to Grade 3", and that non-habitable use such as storage or a plant room should be designed to a minimum of Grade 2. It also states that a Grade 3 system "MUST always include reference to heating and/or ventilation", because condensation is generated inside the room and "will not be eliminated by the application of a structural waterproofing system". A dry cellar that nobody ventilated is still going to feel damp.
Where a drained system is specified, the advice you are given should include the position and type of the internal drainage "including facility for cleaning/inspecting", the position, size, number and type of pumps with their designed extraction capability stated, and the servicing requirements or agreement. If your quote names none of those, it has not been designed, it has been priced.
And then the question worth more than any of this. The PCA imposes a disclosure duty on a contractor quoting a reduced specification. Where a client asks for a partial system, "the implications of this must be fully explained", the limitations "should be clearly stated in the report", and where a quotation is provided for such works "it is advisable to also include an alternative quotation for a specification that complies with BS 8102: 2022, thus giving the client the ultimate choice". So ask for it. If the quote in your hand is a partial system, there should be a compliant alternative beside it. Asking what grade this was designed to, and where the compliant alternative quotation is, tells you within about ten seconds whether anybody designed anything.
On who should be doing that designing, the PCA is direct: "Much of the failure associated with structural waterproofing is attributable to insufficient consideration of the relevant factors, leading to poor or inappropriate design." The qualification to ask about is the Level 3 Certificated Surveyor in Structural Waterproofing, awarded by ABBE and delivered by the PCA, and the awarding body notes that attending the training alone does not entitle anybody to claim it, because the examination has to be passed. There is a further register of Waterproofing Design Specialists beyond that. The PCA also separates the two roles: the designer designs, and installation should be carried out by suitably trained operatives.
What to ask before you sign anything
Six questions. None of them requires you to know anything about waterproofing, and between them they make three quotes comparable.
- Which square metre is that? Wall area, floor area, or both. Until you know, you cannot compare this quote with the next one, and the difference is not small.
- Is the sump, the pump, the chamber and the discharge run inside that rate or outside it? Two published sources use the identical headline rate and answer this differently.
- What grade was this designed to, and who designed it? The grade should come from what you intend to use the room for. If nobody asked you that, nobody chose a grade.
- Where is the compliant alternative quotation? If this is a partial specification, the PCA says one should be offered alongside it.
- Show me the service points on a plan, and tell me who services it. The PCA says the client should be handed a site plan marking the service points, and that this "should be a condition of guarantee". It also admits the channel is very often never serviced, one named reason being that the dry lining gets built out too far forward and obscures access to the very points somebody has to reach every year.
- Two pumps, battery backup, high-level alarm: in or out? All three are what the PCA's own guidance asks for. Any of them missing is a decision somebody made, and you are entitled to have made it.
One more thing worth settling before the work rather than after it. Whether waterproofing an existing cellar needs building regulations approval is genuinely unclear from the regulations themselves, and I am not going to pretend otherwise on your behalf. Requirement C2 covers protection from ground moisture. Regulation 3 lists the parts a material alteration engages and Part C is not among them. Regulation 5 defines a material change of use by what the building becomes, which a cellar inside a house that is already a dwelling arguably does not. Trade sites assert approval is needed and the regulations, read directly, do not obviously agree. Your building control body answers this for your job in one phone call.
About this resource
The technical positions here are the Property Care Association's, quoted from its June 2022 Code of Practice for the waterproofing of existing below-ground structures, its June 2022 Type C best practice guidance, and its June 2022 guidance on groundwater pumping stations. The PCA is a trade body whose members sell this work. I have used it anyway, and said so each time, because it is the only organisation in this market publishing what its own members' systems cannot do. Requirement C2 and the alternative-approach paragraphs come from Approved Document C, and the definitions of material alteration and material change of use from Regulations 3 and 5 of the Building Regulations 2010, all read directly.
On BS 8102, a limit worth stating. The standard sits behind a paywall and I have not read it. What is on this page about it comes from BSI's own catalogue entry: the title, the 31 March 2022 publication date, its current status, and its three-approach scope. Clause-level detail does circulate, but the fullest source for it is a technical paper published by a company that manufactures cavity drain systems, which is one of the two options this page compares. So no clause is quoted here. Worth knowing when a contractor quotes the standard at you, and worth knowing that Approved Document C still points at the 1990 edition.
The prices are every published figure I could find, each attributed to the source that published it and left in that source's own units. Nothing is averaged, because averaging figures that measure different things produces a number describing nothing. Five of the six commercial publishers sell the work or the materials. The two directories ranking at the top of this search both refuse automated access, so nothing is quoted from either; a search snippet quoting a page is not a page I have read. The homeowner figures come from public forum threads and are labelled as one person's experience, not as data.
No annual servicing cost, because none is published anywhere I could reach. No figure for how often battery backup or a second pump is actually fitted, because no such data exists. No verdict on whether your cellar needs building regulations approval, because the regulations do not plainly say and guessing about somebody's legal obligation is not something this site does. No product recommendation on this page either, deliberately: the PCA says half-brick walls and non-structural floors cannot take a cementitious system at all, and the one published DIY worked example in this market is £450 of materials followed by £2,250 of putting it right. England and Wales only. This site does no work, treats no cellars, takes no referral fees and cannot see your wall.
Common questions
What is the difference between tanking and waterproofing?
Tanking is one kind of waterproofing, and treating the two words as the same thing is where most confusion about the price starts. The Property Care Association describes three approaches: Type A, a structure "with no integral protection against water penetration" that "relies totally on a waterproofing membrane to keep water out"; Type B, built with a water-resistant shell, usually reinforced concrete; and Type C, which relies on a drained cavity that collects water and directs it to drainage or a sump. Tanking is Type A. When a quote says "tanking" and another says "waterproofing", they may be pricing two different products with different failure modes and very different running costs.
What is the difference between a tanking membrane and tanking slurry?
Slurry is a cementitious coating bonded to the wall to hold water back. A cavity drain membrane does the opposite: it does not hold water back at all, it lets water arrive behind a studded sheet and manages it away to a sump and pump. The PCA is blunt about the limits of each. Cavity drainage systems "cannot withstand any hydrostatic pressure" and are "only designed to control and manage MINOR leakage and seepage into a structure". Cementitious systems can hold back a significant head of water and need little or no ongoing maintenance, but "all cementitious systems are prone to shrinkage, so cracking and de-bonding is something that will invariably occur to some degree". One has no moving parts and cracks. The other never cracks and depends on a pump and a power supply.
How long does tanking a cellar last?
The answer you get depends entirely on who you ask, and the spread is the most useful thing about it. One contractor page claims 20 to 30 years or longer. Another publishes a warranty of 10 to 20 years. Homeowners on forums who have had it done report about ten years, and describe paint flaking afterwards. Nobody publishes a study, so there is no number here worth standing behind. What is worth knowing is that on a drained system the length is conditional: the PCA states that any failure resulting from a deviation from the planned maintenance programme "will not be the responsibility of the installer and will not be covered by any guarantee that has been issued".
Can I tank a cellar myself?
The materials are cheap enough to make it tempting. One supplier works its own slurry out at roughly £9.08 per square metre for two coats, and £619.50 excluding VAT for a worked walls-and-floor example. The reason to stop and think is what the PCA says about which walls can take it at all: "Non-structural floors and half brick walls are not suited to the application of a cementitious system due to their inability to withstand bending stress", and success "depends on the bond achieved between the material and the substrate", which may need considerable surface preparation. The one worked failure published anywhere in this market is a £450 DIY application that then cost £2,250 to put right. The material is not the risk. Applying a barrier system to a wall that cannot take one is.
What is the British standard for basement waterproofing?
BS 8102:2022, "Protection of below ground structures against water ingress. Code of practice", published on 31 March 2022 and currently active. BSI describes its scope as three approaches: barrier materials applied to the structure, structurally integral watertight construction, and drained cavity construction. Two things are worth knowing before anyone quotes it at you. It is a code of practice rather than law. And Approved Document C, the government guidance a builder actually works to, still points at BS 8102:1990, an edition two revisions out of date.
Do you need building regulations approval to tank a cellar?
This deserves a straighter answer than it usually gets, and part of the straight answer is that I cannot give you one. Requirement C2 of the Building Regulations says walls and floors "shall adequately protect the building and people who use the building from harmful effects caused by ground moisture". But under Regulation 3 the requirements engaged by a material alteration are Parts A, B1, B3, B4, B5, M and T, and Part C is not among them; and a material change of use under Regulation 5 turns on the building becoming something it was not, which a cellar inside an existing house arguably does not. Trade sites assert approval is needed. The regulations, read directly, do not obviously say so. Your building control body settles this for your specific job, and it is worth one phone call before the work rather than after it.