Non-Standard Construction Mortgages: Which Lenders Say Yes

Written by Jake Stott, Knox Mortgages. Published 23 September 2026. Last reviewed 23 September 2026.

A non-standard construction mortgage is a mortgage on a home that was not built from traditional brick or stone walls under a pitched tiled or slate roof. That covers concrete, steel and timber systems, prefabricated houses, Mundic block, single skin brick, thatch and flat roofs. Lenders assess these properties individually, and some types are declined outright unless they have been repaired and certified.

The short answer to the question most people are really asking: yes, most non-standard homes can be mortgaged, but the construction type decides which lenders will look at it, what paperwork the valuer needs, and whether the case survives valuation. This page sets out who lends on what, based on the lenders’ own published criteria.

Which non-standard construction types can you get a mortgage on?

It depends on the system, not on the label “non-standard”. The table below is the quick reference. It shows whether a type is designated defective under Part XVI of the Housing Act 1985 (which consolidated the Housing Defects Act 1984), what mainstream lenders typically require, and the realistic route to a mortgage.

Construction type	Designated defective?	What lenders typically require	Realistic route
Repaired PRC (Airey, Cornish Unit, Orlit, Reema, Unity, Woolaway and others)	Yes	PRC certificate from a licenced repair scheme	Selected high street lenders and specialists
Unrepaired PRC	Yes	Declined by mainstream lenders	Cash, or bridging then repair and refinance
BISF (British Iron and Steel Federation) steel house	No	Varies sharply; NatWest declines, reclad or not	Selected lenders and specialists
Wimpey No-Fines and Laing Easiform concrete	No	Often accepted, subject to valuation	Many high street lenders
Timber frame, post-1965, brick or block clad	No	Usually treated close to standard	Most of the market
Mundic block (Cornwall and Devon)	No	Mundic test report graded Class A	Depends entirely on the grade
Modern Methods of Construction (modular, SIPs, CLT)	No	Recognised accreditation such as BOPAS, NHBC Accepts or LPS 2020	Accreditation dependent
Single skin brick, steel frame, glass	No	Case by case, often a structural engineer’s report	Specialist lenders

What is considered non-standard construction?

Non-standard construction is any building method other than cavity brick or block walls with a conventional roof. Most of it dates from the post-war housing drive of the 1940s to the 1960s, when councils needed homes quickly and steel, concrete and timber systems filled the gap left by a shortage of bricks and bricklayers.

The main families are:

• Pre-cast reinforced concrete (PRC): factory-made concrete panels or columns. Airey, Cornish Unit, Orlit, Reema, Unity, Woolaway, Tarran, Smith, Boot, Wates, Parkinson and Stent are all PRC systems.
• In-situ concrete: concrete poured on site, such as Wimpey No-Fines and Laing Easiform.
• Steel frame: BISF houses and other steel systems.
• Timber frame: from 1920s prefabs to modern brick-clad timber homes.
• Mundic block: concrete blocks made with mine waste aggregate, found in Cornwall and parts of Devon.
• Modern Methods of Construction: modular, structural insulated panels and cross-laminated timber.

Many of these homes were built as council housing, so a large share now sits in ex-local authority stock bought under Right to Buy. Estate agents are not always clear about construction type, so ask before you offer.

What does “designated defective” mean under the Housing Defects Act?

A designated defective dwelling is a house type the Secretary of State formally declared defective because of its design or construction, where public knowledge of the defect had substantially reduced values. The power came from the Housing Defects Act 1984 and now sits in Part XVI of the Housing Act 1985, section 528.

The designations were aimed at PRC houses. In the late 1970s and early 1980s, carbonation of the concrete and chloride in the mix were found to corrode the steel reinforcement, cracking the concrete around it. Designation was made by individual notice rather than in one schedule, and industry lists put the total at around 35 types in England and Wales, including Airey, Cornish Unit Types 1 and 2, Orlit, Reema Hollow Panel, Unity, Woolaway, Tarran, Smith, Boot, Wates, Myton, Newland, Dorran and Gregory. Scotland has its own list under the Housing (Scotland) Act 1987.

Two points matter for buyers. BISF, Wimpey No-Fines and Laing Easiform were not designated. And designation does not stop a house being mortgaged once it has been properly repaired. It stops it being mortgaged while it has not.

What is a PRC certificate and why do lenders need one?

A PRC certificate confirms that a designated PRC house has been structurally repaired under a licenced repair scheme and signed off by an approved inspector. Without one, mainstream lenders treat a designated PRC house as unmortgageable. NatWest, for example, states that unrepaired PRC is unacceptable regardless of its Housing Defects Act status.

Licenced repair schemes are administered by PRC Homes Ltd. The repair normally replaces the failing concrete outer walls with a traditional brick or block skin, supervised by a qualified structural engineer, and ends with a certificate of structural completion. NatWest goes further than some lenders: it accepts repaired PRC only where the walls have been totally rebuilt in traditional materials with a PRC Homes guarantee, and only for houses, not flats.

Three practical checks before you offer:

• Ask the seller for the original certificate, not a photocopy of a letter referring to it.
• Check the repair was a licenced scheme, not a cosmetic overclad or render.
• On a semi-detached or terraced PRC, ask whether the attached neighbour was repaired too. Lenders and valuers treat an unrepaired attached half differently, so raise it early.

Which lenders offer non-standard construction mortgages?

Most high street lenders will consider some non-standard types, but only a few publish detail. The table below reflects each lender’s own intermediary criteria, last reviewed 23 September 2026. Where a lender does not publish specifics, we say “case by case” rather than guess. Criteria change, so treat this as a starting point, not a decision.

Lender	Published position
NatWest	Repaired PRC houses with walls rebuilt in traditional materials and a PRC Homes guarantee; unrepaired PRC declined. BISF declined, reclad or not. Wimpey and Scottish no-fines accepted. Post-1965 brick or block clad timber frame accepted. Steel frame houses with conventional cladding accepted. Mundic needs a Class A report under six years old.
Nationwide	Accepts timber frame and many Modern Methods of Construction. Non-traditional types go through a pre-valuation form, and variants of the same system can be treated differently.
Halifax	Modern Methods of Construction need BOPAS, NHBC Accepts or LPS 2020 approval, or a 60 year design life. Older PRC and steel types assessed case by case with the valuer.
Barclays	Modern Methods of Construction need individual valuer assessment. Early steel frame not normally suitable, but may be considered with a structural engineer’s report.
HSBC	Publishes a non-standard construction list to brokers; assessed case by case.
Santander	Case by case, subject to valuation.
Accord Mortgages	Mundic Class A accepted; A/B referred to the valuer; Class B and below declined.
United Trust Bank	Specialist. Explicitly considers concrete, single skin brick, steel framed, timber framed and glass construction.

Building societies such as Darlington, Mansfield and Buckinghamshire also consider non-standard homes through manual underwriting. They are often where a case lands after the high street says no.

How much deposit do you need for a non-standard construction house?

There is no single figure. None of the lenders reviewed publishes a blanket loan to value cap for non-standard construction. What actually happens is binary: an acceptable type, properly certified, is usually lent on at or close to normal loan to value bands, while an unacceptable type is declined at any deposit.

The deposit risk sits in the valuation, not the product rules. Valuers routinely value non-standard homes below comparable brick houses, and the lender lends against the valuation, not the price you agreed. Build a buffer into your plans. A buyer with 10% who might need 15% after a cautious valuation is in a far stronger position than one who has nothing left. Our guide to how much you can borrow covers the affordability side.

What happens if the valuer downvalues or declines the property?

A downvaluation means the lender will lend less than you expected; a decline means that lender will not lend at all. Both usually happen at the valuation stage, after you have paid for searches and a survey. This is the point most non-standard cases die, and it is almost always avoidable with the right lender chosen at the start.

Worked example: a repaired PRC semi

This is an illustrative example, not a client case. A buyer agrees £180,000 for a repaired Airey semi and plans a 90% mortgage of £162,000 with an £18,000 deposit.

• The valuer values it at £165,000, noting the construction type.
• At 90% of £165,000, the maximum loan is £148,500.
• The shortfall is £162,000 minus £148,500, which is £13,500.
• Options: find £13,500 more deposit, renegotiate the price to the valuation, or move to a lender whose valuer takes a different view.

If the valuer declines instead, appealing rarely works. Moving the application to a lender that accepts the system does. Our page on mortgage valuations explains how valuations work in more detail.

What survey do you need on a non-standard construction house?

Get a RICS Level 3 survey at minimum, and expect a structural engineer’s report on PRC, early steel frame or anything the valuer flags. The lender’s valuation is not a survey. It protects the lender, not you.

Report	Typical 2026 cost	When you need it
RICS Level 3 building survey	£600 to £1,500, most homes £700 to £950	Every non-standard purchase
Structural engineer’s report	£500 to £2,000, defect-specific reports around £600 to £1,000	PRC, steel frame, cracking, or when requested by the lender
Mundic test report	Varies by laboratory and sample count	Pre-1950s homes in Cornwall and Devon

Survey costs from HomeOwners Alliance and Checkatrade 2026 guidance. Allow extra time too. Engineer reports and certificate checks commonly add two to four weeks before a mortgage offer.

Is it worth buying a non-standard construction house?

It can be, if the type is widely lendable and the price reflects the construction. A certified repaired PRC house, a Wimpey No-Fines semi or a brick-clad timber frame can be good value, especially for first-time buyers and landlords. An unrepaired PRC house or a BISF at a full brick-house price usually is not.

The test is your future buyer. If the next purchaser cannot get a mainstream mortgage, your sale depends on cash buyers and specialist lending, which means a slower sale and a lower price. Check insurance before you commit too: some insurers load premiums or refuse certain systems.

For investors, cheap unmortgageable stock can work as a buy, refurbish, refinance project: buy with bridging finance, complete a licenced repair, then refinance onto a buy-to-let mortgage. The numbers must work after repair costs, which often run into tens of thousands.

Is it hard to sell a non-standard construction house?

It is harder than selling a brick house, but how much harder depends on the type. Widely accepted systems sell to mortgaged buyers much like standard homes. Types most lenders decline sell mainly to cash buyers, investors and specialist borrowers, so expect longer marketing periods and sharper offers. Keep every certificate, engineer’s report and guarantee: a complete paper trail is what lets your buyer’s lender say yes.

Frequently asked questions

Is it possible to get a mortgage on a non-standard construction property?

Yes, in most cases. Timber frame, Wimpey No-Fines, Laing Easiform and repaired PRC with a certificate are all lent on by mainstream lenders. Unrepaired designated PRC and some steel types, including BISF at some lenders, are declined by the high street and need a specialist or cash.

What is considered non-standard construction?

Any home not built with brick or block cavity walls and a conventional pitched roof. That includes concrete, steel and timber systems, prefabs, Mundic block, single skin brick, thatch, flat roofs and Modern Methods of Construction such as modular homes.

Do non-standard construction mortgages have higher rates?

Not always. If a mainstream lender accepts the type, you usually get its normal rates. Higher rates apply when the case needs a specialist lender because the high street has declined the construction type.

Which high street lenders accept BISF houses?

Fewer than people expect. NatWest declines BISF, reclad or not. Other lenders assess BISF case by case, often requiring a structural engineer’s report. Check the specific lender’s position before you pay for a valuation.

Can you get a mortgage on an Airey house?

Yes, if it has been repaired under a licenced scheme and has a PRC certificate. Airey is a designated defective type, so an unrepaired Airey house is generally unmortgageable with mainstream lenders.

Can you get a mortgage on a Wimpey No-Fines house?

Yes. Wimpey No-Fines is not designated defective, and lenders including NatWest accept it. It is still non-standard, so the valuer will comment on condition, particularly cracking, render and wall ties.

What Mundic grade do lenders accept?

Class A. Accord accepts Class A, refers A/B to the valuer and declines Class B and below. NatWest wants a Class A report under six years old. The current guidance is the RICS Mundic Problem, 3rd edition.

Can I get a non-standard construction mortgage with bad credit?

Yes, but the lender pool is smaller because you need a lender that accepts both the property and the credit history. Specialist lenders are usually the answer. See our guide to bad credit mortgages.

Is there a non-standard construction mortgage calculator?

No calculator can tell you whether a lender accepts a construction type, which is the question that decides the case. Standard affordability calculations still apply once a lender accepts the property. Get an agreement in principle with a lender that accepts the type before you offer.

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