Non-Standard Construction and Mortgages: RAAC, PRC Concrete and Steel-Frame Homes

Why construction type matters so much to lenders

Most UK homes are built from traditional brick, block or well-understood modern timber-frame systems, and mainstream lenders mortgage these routinely without a second thought. A smaller but significant number of properties, mostly built in specific waves of post-war and later construction, use methods that have since revealed structural weaknesses, and these properties can be far harder — sometimes currently impossible — to mortgage until specific conditions are met.

RAAC: a lightweight concrete with a known failure mode

Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight form of concrete, cheaper and quicker to produce than traditional concrete, used mainly in flat or low-pitch roof panels across a range of building types — including schools, public buildings, and some residential properties — roughly from the 1950s through to the 1990s.

RAAC gained significant public attention after structural failures highlighted a specific, serious risk: as it ages, RAAC can fail suddenly and without extensive prior warning signs, unlike the more gradual deterioration typical of traditional concrete or masonry.

PRC concrete: a broader category with a formal defective list

Precast Reinforced Concrete (PRC) refers to a range of post-war house-building systems that used factory-made concrete panels instead of traditional brick construction, developed partly to address housing shortages quickly and cheaply. Various specific PRC house types (with names like Airey, Cornish Unit, Wates, and others) were built in significant numbers.

A number of these specific PRC systems were later found to suffer from corrosion of the reinforcement within the concrete panels, weakening the structure over time, and were formally designated as "defective" under the Housing Act 1985.

Getting a mortgage on a designated defective PRC property

The good news: many of these house types can still be mortgaged today, but generally only where:

1. The property has undergone approved structural repair under a recognised licensed repair scheme specific to that house type
2. The repair has been certified, with documentation available to demonstrate this to a lender
3. The lender's own valuer/surveyor confirms the repair certification is in order

Without this certification, most mainstream lenders will decline to lend, since the underlying structural risk the "defective" designation identified hasn't been formally addressed.

Non-standard construction that's usually fine

It's important not to over-generalise: a great deal of non-standard construction — a term simply meaning "not traditional brick and block" — is perfectly well understood and mortgageable, including:

• Modern timber frame construction (a substantial proportion of new-build homes)
• Modern steel frame systems used by many contemporary developers
• Cross-laminated timber (CLT) and other modern engineered construction methods

The specific concern for lenders is around older systems with known, documented structural weaknesses — principally RAAC and formally defective PRC designations — not non-standard construction as a general category.

How to find out if a property is affected

A RICS Level 2 (Homebuyer) or Level 3 (Building) survey should identify or at least flag suspicion of non-standard construction, and a competent surveyor familiar with the local housing stock (particularly relevant for post-war estates) should recognise specific PRC house types by their characteristic appearance. Your conveyancing solicitor's searches may also surface any historical defective housing designation attached to the property.

Practical steps for buyers

1. Instruct at least a Level 2 survey, and a Level 3 if the property's age or appearance raises any suspicion of non-standard construction.
2. Ask the seller/agent directly whether the property has, or has ever had, a defective housing designation, and request any repair certification documentation if so.
3. Speak to your mortgage broker early if construction type is uncertain — some lenders have specific, published lists of acceptable and unacceptable non-standard construction types.
4. Budget time for a structural engineer's report if RAAC is suspected, since this is often a prerequisite before any lender will proceed.

Bottom line

Not all non-standard construction is a mortgage problem, but RAAC and formally defective PRC concrete systems are genuine, well-documented exceptions that can make a property very difficult or currently impossible to mortgage without specific structural certification. A thorough survey pitched at the right level, combined with early conversations with a broker familiar with non-standard construction cases, is the best protection against discovering this problem only after you've committed to a purchase.