The waterproofing checklist for buying a London home with a basement

A Documents to request from the seller's solicitor

The O&M manual & waterproofing design package

• Full waterproofing design report (a "design philosophy" report), ideally prepared by a competent Waterproofing Design Specialist.
• All waterproofing drawings — plans, sections and details, including the critical junctions: construction joints, movement joints, service penetrations, and podium / landscaping interfaces.
• A clear written explanation of which waterproofing systems are installed, and where.
• Identification of the waterproofing type(s) — and whether more than one was used. A single system in a high-value habitable basement is a single point of failure.Type A: barrier (slurry or membrane). Type B: structurally integral (water-resistant concrete). Type C: drained cavity system.
• The Grade the space was designed to achieve versus how you intend to use it. Turning a damp-tolerant store into a fully dry habitable room is a design change, not a decorating job.
• Ground investigation & water table — is the basement permanently below the water table, permanently dry, or variable? Plus any aggressive-ground findings (sulphates, contamination, ground gas, radon).

Warranties, guarantees & the people behind them

• Workmanship / installation warranties — and read the terms, not just the certificate:
  • A clear statement of what work was actually completed.
  • Confirmation that payment was received in full, so the warranty stands.
  • Confirmation the warranty transfers to you — and any fee, process or time limit.
• Manufacturer / product warranties — and confirmation the product was fitted by an approved applicator. These are frequently void if it wasn't.
• Insurance-backed guarantee — original certificate, scope of works, confirmation of payment in full, and confirmation the underwriter is still operating.
• Structural / latent-defects warranty (NHBC, LABC, Premier, etc.) — read the below-ground exclusions specifically. Many limit or exclude basement waterproofing.
• The designer's professional indemnity — is the original designer still trading, or is run-off cover in place? It matters if a claim is ever needed.
• Who designed it and who installed it — named specialist contractor, PCA membership, evidence of competence.

Maintenance & condition history

• Maintenance records for any cavity drainage system — how often, by whom, and the date of the last service.
• History of any leaks, damp, water ingress or insurance claims relating to water, for this property.
• Records of any remedial waterproofing works (e.g. resin injection) — and whether those works were themselves designed and warranted.

B The independent inspection to commission

Don't rely on the seller's documents alone, and don't rely on a standard survey. Commission your own specialist before exchange.

• The obvious one: is it leaking? And beyond active leaks — a condition check for cracking, efflorescence and salts, damp patches, condensation and musty smell.
• CCTV survey of all perimeter drainage channels, with confirmation of free flow throughout the system.
• Check the channels for build-up — silt, calcite and especially iron ochre, a well-known killer of cavity systems and a sign of trouble ahead.
• Accessibility for maintenance. Are the channels, inspection ports and sump still reachable — or buried under finishes, screed or fixed cabinetry? In a luxury fit-out this is one of the most common and costly problems.
• An independent report on the sump, pump and discharge:
  • Multiple pumps (duty / standby) in good condition, each with its own non-return valve.
  • Power-loss backup — battery at minimum, ideally a second independent supply or generator — plus a high-level alarm.
  • Electrical arrangement compliant with current good practice. Note when the basement was built — older installs may pre-date current standards.
  • Where does it discharge, and is the route adequate and consented? Check for a non-return valve against sewer surcharge / backflow — a real risk on London's combined sewers.
  • Telemetry, monitoring or pump run-time data? Pump cycles reveal how much water the system is actually handling, and the trend.
• Establish a baseline before you commit. Consider temporary monitoring before exchange — water level, pump cycles, moisture — while the property is still the seller's responsibility.

C External & historical factors that may have compromised the design

• When was the basement built? Older basements may pre-date current standards — relevant to both the waterproofing strategy and the pump and electrical compliance above.
• Neighbouring works since construction — adjacent basement digs, infrastructure groundworks, and especially dewatering, which can mask the true water table. When the dewatering stops, ingress can appear that was never there during the seller's ownership.
• Party Wall agreements for any nearby works — these can remain relevant for 10 or 20 years, and may carry rights and obligations that pass to you.
• Landscaping works since construction — have any affected the waterproofing or drainage to the podium or buried decks, particularly where these extend beyond the building footprint?
• Trees removed or planted near the structure. In London clay, shrinkage and heave can affect both structure and waterproofing.
• Nearby drainage and utilities — leaking drains or sewers raise the local water table and load the waterproofing. Note any history of sewer surcharge in the area.
• Flood risk — surface-water and groundwater flood risk for the area (Environment Agency maps). The basement is the lowest point and floods first.
• Change of use — has a plant room or car park become a habitable cinema, gym, wine room, pool or spa? Habitable use demands a higher Grade; a pool or spa adds a moisture and condensation load the original design may never have anticipated.