10 Common Building Defects Found in London Properties

In the past five years, I have surveyed well over 300 properties across London. In that time, I have found some genuinely alarming things hiding behind freshly painted walls, under new carpets and above newly plastered ceilings. The London housing stock is old — a significant proportion of it was built before 1939 — and older buildings come with older problems.

Here are the ten defects I most commonly encounter, what they mean for the property and what you should do if your survey flags them up.

1. Rising Damp

Rising damp occurs when moisture from the ground rises up through the walls by capillary action. It is most common in properties where the damp-proof course (DPC) is absent, has failed, or has been bridged by raised ground levels or piled-up soil. Signs include tide marks at low level, salting, crumbling plaster and a musty smell.

What it means for buyers: Rising damp treatments can range from £1,500–£5,000 per affected wall, depending on severity. Be cautious: this is also one of the most over-diagnosed and over-sold remediation products in the industry. Always get a second specialist opinion before spending money on chemical injection treatments.

2. Penetrating Damp

Penetrating damp is water ingress through the external envelope — walls, roof, windows or joints. Common causes include failed pointing, cracked render, blocked gutters, defective flat roofs and failed window seals. Unlike rising damp, it can appear at any height in the building.

What it means for buyers: The repair cost depends entirely on the cause. A blocked gutter is a £150 fix. A failed flat roof covering may cost £8,000–£20,000 to replace.

3. Structural Cracking and Movement

Cracking in walls is incredibly common in London's Victorian and Edwardian housing stock, and not all of it is serious. Fine hairline cracks in plaster are typically cosmetic. Wider diagonal cracking in brickwork, or cracks that are tapered or stepped through the mortar joints, may indicate differential settlement or structural movement requiring investigation.

What it means for buyers: Minor cracking may be £500–£2,000 to cosmetically repair. Serious structural movement, particularly subsidence, can cost £20,000–£100,000+ to remedy.

4. Subsidence

London's clay soils are particularly susceptible to shrinkage during dry summers, causing foundations to move and the building above to crack. Tree roots are a common culprit — particularly large trees close to older properties with shallow foundations. Subsidence claims are common in London, and properties with a history of subsidence may be difficult or expensive to insure.

What it means for buyers: Check whether the property has a history of subsidence claims. Ensure your survey includes a specific assessment of any cracking and, if necessary, instruct a structural engineer.

5. Defective or End-of-Life Roofs

The roof is the most expensive single element in most houses, and it is the one most commonly underestimated by buyers. Defective flashings, missing or slipped slates and tiles, failed flat roof coverings and deteriorating lead work are all extremely common in London's Victorian and Edwardian stock.

What it means for buyers: A complete re-roof can cost £8,000–£25,000. Even partial repairs can run to several thousand pounds. Always budget for roof works on older properties.

6. Woodworm, Dry Rot and Wet Rot

Timber defects are common — particularly in properties where damp has been allowed to penetrate over time. Woodworm (furniture beetle larvae) creates networks of bore holes in floor boards, joists and roof timbers. Dry rot is a fungal attack that can devastate structural timbers in weeks if conditions are right. Wet rot is less destructive but indicates a persistent source of moisture.

What it means for buyers: Treatment and structural repairs can range from a few hundred pounds (localised woodworm) to £20,000+ (extensive dry rot outbreak). Always get a specialist timber and damp survey if your main survey flags up any concern.

7. Cavity Wall Tie Failure

Properties built between approximately 1920 and 1980 with cavity walls often used metal wall ties that are now corroding. As they rust, the ties expand and can cause horizontal cracking through the mortar joints (typically at regular vertical intervals). This defect is common in post-war stock and is often underestimated by buyers.

What it means for buyers: Wall tie replacement typically costs £2,500–£8,000 depending on the extent of the problem. Early intervention prevents significantly greater structural damage.

8. Flat Roof Defects

Flat roofs over bay windows, extensions and garages have a finite lifespan — typically 15–25 years depending on the covering material. Failed felt, ponding water, cracking asphalt and inadequate drainage are all common findings. In leasehold properties, flat roof maintenance is often a contentious service charge issue.

What it means for buyers: A new flat roof covering typically costs £1,500–£5,000 per area, depending on size and the specified material.

9. Inadequate Electrical Installations

Older London properties frequently have outdated electrical installations — rubber-insulated wiring, rewirable fuse boards, inadequate earthing and circuits that do not comply with current Part P regulations. These are a genuine safety risk and may be flagged as requiring immediate investigation by a qualified electrician.

What it means for buyers: A full rewire of a typical London terraced house typically costs £4,000–£8,000. Partial upgrades may be significantly less, depending on the extent of the work required.

10. Defective Drainage

Blocked, cracked or collapsed drains are surprisingly common in older London properties. Root ingress, ground movement and decades of use all take their toll. A CCTV drain survey is not included in a standard building survey, but our surveyors will always flag evidence of drainage problems and recommend one where appropriate.

What it means for buyers: Minor drain repairs can be a few hundred pounds. A collapsed drain requiring excavation and relining can cost £3,000–£15,000.

FAQs About Building Defects