Non-Standard Construction Mortgages

What is a non-standard construction mortgage?

A non-standard construction mortgage is an ordinary residential or buy to let mortgage on a home that the lender's own property criteria do not class as standard construction. There is no single definition, because each lender keeps its own list. Coventry Building Society, for example, expects brick or stone walls with a tile or slate roof, and accepts timber or steel frame only where the cladding is mostly brick or stone, while Nationwide, The Mortgage Works and Virgin Money treat thatch and modern timber frame with a masonry outer wall as standard or traditional construction. The same home can be standard at one lender and non-standard at another.

The decision turns on the detail of the individual home. Nationwide's criteria say the exact construction name matters because lending terms can differ between types and the years they were built, and NatWest says constructions it lists as acceptable are still subject to its valuer confirming the property is suitable security. No named system is mortgageable, or unmortgageable, across the whole market: it is accepted, considered with conditions or declined lender by lender.

The lender examples on this page come from each lender's own published criteria, most of them written for mortgage intermediaries, checked on 29 September 2026 unless another date is shown. They illustrate how rules differ and can change at any time. They are not a list of the lenders Mortgage Lane works with, and not a decision on any application.

What lenders and valuers establish

Before a lender decides, it and its valuer establish what the home is, what has been done to it, and whether it will sell. The same questions come up for almost every non-standard home:

• The exact system and build year: a general description such as concrete or steel is not enough. Nationwide sets different terms for variants of one system, such as Gregory, Gregory Drury System 3 and Gregory Housing, and offers a pre-valuation form so the construction can be checked first. Many lender rules for timber and steel frame turn on the year the home was built.
• Repair certification: for a precast reinforced concrete (PRC) home, which scheme repaired it, whether a certificate was issued at the time, and whether the lender accepts that certificate. West Bromwich Building Society, for example, does not accept retrospective certificates (lending and security guidelines, August 2026).
• The adjoining property: for a semi-detached or terraced PRC home, whether the attached homes were repaired to the same standard, which several lenders require.
• Cladding and the outer walls: what the outer skin is made of, and how much of it. Coventry Building Society needs cladding that is predominantly (67%) brick or stone on the timber and steel frame homes it accepts.
• Warranty and accreditation: a structural warranty for a newer home, BOPAS accreditation for modern methods of construction, and, for some later steel frame homes, a BBA (British Board of Agrément) or WIMLAS certificate, which are independent certifications of the building system.
• Buildings insurance: whether cover for the construction is available. The Cumberland Building Society will not lend on non-standard construction where insurance covering the construction type cannot be obtained (criteria guide, June 2026).
• The valuer's saleability view: the valuer reports on condition and on whether the home will sell. Together, a specialist lender, counts a property with poor valuation remarks as non-standard property in its intermediary product guide dated 23 September 2026.
• Residential or buy to let: a lender's buy to let rules for a construction type can differ from its residential rules, so the policy for the intended use is the one that counts.

Some lenders decline most named systems. Vernon Building Society, for example, is unable to consider Cornish, Woolaway, BISF, PRC (repaired or unrepaired), large panel system, steel frame, single skin, a majority flat roof, deck access or spray foam, although it accepts thatch subject to a satisfactory valuation (criteria checked 29 September 2026).

If you are buying or remortgaging a non-standard home, get in touch with the address and any survey, repair certificate, warranty or structural report you hold. The construction is identified before a lender or a valuation is chosen.

How much can you borrow on a non-standard home?

There is no market-wide maximum loan to value (LTV) for non-standard construction: the maximum depends on the lender, the product and the valuation. Most lenders' published construction policies give no separate maximum for each type. The few construction-specific limits found are each one lender's rule:

• Together caps property it classes as non-standard at 65% LTV in its intermediary Personal Finance Product Guide dated 23 September 2026, and says a full assessment can reduce that further. Its Network and Clubs guide of the same date applies the same 65% to its unregulated buy to let range. Together's intermediary property type index lists 65% for systems including Cornish, BISF, Woolaway, steel frame, single skin brick and block work and thatch, up to 75% for timber frame, and 75% for flat roofs, flats with deck access and spray foam under the roof. The published documents do not say how those index entries sit alongside the 65% cap. Which figure applies depends on whether Together classes the particular property as non-standard, on the product and on its valuation and full assessment.
• HSBC considers period (older, traditionally built) timber-framed homes and those built after 1965 subject to the valuer's comments, with a maximum LTV of 80% (intermediary FAQ).
• Kensington accepts No Fines and Laing Easiform houses (types of concrete poured on site rather than precast) to a maximum 80% LTV (Property Exclusions, June 2026).
• Metro Bank requires standard cavity or solid masonry construction with a pitched tile or slate roof, and no more than 25% flat roof, for loans above 95% LTV (residential lending criteria, July 2026).

An older Together criteria document that is still online, but undated, gives 60% for non-standard property. This page uses 65% because Together's guides dated 23 September 2026 state that they supersede all previous packs and criteria.

Illustrative arithmetic only, on an assumed purchase price and valuation both of £200,000: a 65% maximum allows a loan of up to £130,000, so a deposit of at least £70,000; a 75% maximum allows up to £150,000, so at least £50,000. If the valuation came in lower than the price, the loan would be worked out on the lower figure and a larger deposit would be needed. These are not products or offers. The lender's valuation, product and affordability assessment decide the actual figure.

Rates depend on the lender and product at the time of application, so none is quoted here. Where the construction narrows the choice of lender, an adviser checks which lenders' current criteria accept it before comparing the products available.

Non-standard construction mortgage criteria

The table sets out, for each common type, what a lender and its valuer establish and how published lender rules differ. It has no loan to value column, because no cross-lender maximum exists for any construction type: the few published limits are single lenders' rules, given in the previous section and, with other named and dated lender examples, in the sections below.

Concrete homes: PRC, Cornish, Woolaway and large panel system

Precast reinforced concrete (PRC) homes are judged on the exact system, whether it was repaired under an approved scheme, and whether the homes attached to it were repaired too.

Designated defective homes

In England and Wales, Part XVI of the Housing Act 1985 lets the government designate classes of building as defective by reason of their design or construction (section 528). The national list for England, given in Parliament on 25 July 1988, includes Cornish Unit and Woolaway alongside Airey, Orlit, Reema Hollow Panel, Unity and Butterley, Wates and others. Designation is a legal status. Whether a lender will lend is a separate question under its own criteria.

Unrepaired PRC

Most lenders that publish a policy decline unrepaired PRC. NatWest declines it whether or not it is designated defective, and Aldermore declines unrepaired PRC and asks for a repair certificate (property and valuation policy guide, June 2026). Together's intermediary property index is an exception: it refers non-repaired PRC case by case.

Repaired PRC and the repair certificate

A repair certificate shows that a home was repaired under a licensed or registered scheme. Many earlier repairs ran under the PRC Homes Ltd scheme, run by a subsidiary of NHBC (the National House Building Council). BRE Global's scheme issued its first certificate in July 2016, has registered repair specifications for Airey, Reema, Underdown/Winget and Wates houses, and keeps a register of repaired properties that lenders use (BRE PRC repair scheme). Lenders then add their own conditions. NatWest accepts repaired PRC houses only where the walls were totally rebuilt in traditional materials, PRC Homes Ltd issued a guarantee and all houses in the structural block were improved to the same standard, and it declines PRC flats. Virgin Money considers PRC repaired under an approved scheme for homes of no more than two storeys with no adjacent unrepaired property. Some lenders decline PRC even when repaired, including Kensington (June 2026).

A repaired semi attached to an unrepaired home

Several lenders publish a rule for exactly this case. Accord Mortgages treats a PRC flat, or a home adjoined to an unrepaired PRC property, as unacceptable regardless of any report. Virgin Money requires that there is no adjacent unrepaired property, NatWest needs the whole structural block improved, Aldermore requires adjoining properties to have been correctly repaired, and The Mortgage Works says its terms vary with the construction type and any repair scheme, and asks whether the scheme covers the whole block, meaning the whole terrace or both semis. A repaired semi next to an unrepaired half can therefore be declined even with a valid certificate for the repaired half.

Cornish and Woolaway homes

Cornish Unit and Woolaway are designated PRC systems in England. Neither is among the house types with a registered BRE Global repair specification, so lenders look for evidence of a repair under an earlier licensed scheme. Lenders differ sharply: West Bromwich Building Society considers repaired Cornish Unit and Woolaway homes where the valuer supports the case, the repair was under an approved scheme and the immediately adjacent home was also repaired, with a PRC Homes Ltd certificate or an engineer's confirmation that the scheme was licensed before September 1996 (August 2026). Metro Bank declines homes designated defective whether repaired or not (July 2026). Together's index lists 65% for both, subject to its classification, product and full assessment.

Concrete panel and large panel system

Large panel system (LPS) construction is declined by many lenders, including Virgin Money, Aldermore, Kensington and Metro Bank, and by SBI UK for buy to let. NatWest considers LPS houses with a satisfactory report from a structural engineer, but not LPS flats or maisonettes. Other concrete systems are treated separately: Kensington excludes LPS but accepts No Fines and Laing Easiform houses to a maximum 80% LTV. Ex-council blocks add their own tests, covered on our mortgage on an ex-council flat page. For landlords, see PRC construction mortgages and concrete construction houses mortgages.

Steel, timber and single skin homes: BISF, steel frame, timber frame and Colt

Steel, timber and single skin homes are judged on the system, the build year and what the outer walls are made of.

BISF houses

BISF (British Iron and Steel Federation) houses are a steel system, not a designated PRC type, but many lenders still restrict them. NatWest does not accept BISF whether or not it has been reclad, and SBI UK lists BISF as unacceptable for buy to let. West Bromwich Building Society permits pre-1990 steel systems, including BISF, subject to the valuer's comments and a full structural survey (August 2026), and Together's index lists 65%, subject to valuation and to Together's classification, product and full assessment.

Other steel frame homes

The build year and certification decide most steel frame cases. Aldermore considers steel frame built after 1987 with BBA or WIMLAS certification (independent certification of the system) and declines earlier steel-framed houses (June 2026). TSB's buy to let criteria ask for a structural engineer's report for steel frame built before 1987, and a WIMLAS or BBA certificate or an engineer's report after 1987. Coventry Building Society accepts steel frame built on or after 1 January 1990 with cladding predominantly brick or stone, and Virgin Money treats modern steel frame with a masonry outer skin as standard if it originally had an NHBC guarantee. Kensington excludes steel framed or clad houses.

Timber frame

Many lenders accept modern timber frame with masonry outer walls, but the year thresholds differ. Coventry Building Society accepts timber frame built after 1945, with the 67% brick or stone cladding rule. NatWest accepts softwood timber frame built since 1965 with brick, reconstituted stone or rendered blockwork cladding, and HSBC declines timber frame built between 1920 and 1965, although it has a separate kit-built category for named systems such as Colt and Guildway (see below). Nationwide and The Mortgage Works treat masonry-clad timber frame built in 1970 or later as traditional construction, and Vernon Building Society accepts timber frame built after 1970. Kensington excludes 100% timber frame unless it was built after 1980 and is of modern construction. All-timber or timber-clad homes are often declined: Virgin Money declines timber cladding unless it is decorative, Aldermore declines 100% timber, and SBI UK lists timber clad properties as unacceptable for buy to let.

Colt and Guildway homes

Colt describes its houses as timber-framed homes prefabricated in its factory, so timber frame and cladding rules apply alongside any rule for the named system. Lenders differ widely: NatWest treats pre-1965 softwood timber frame as unacceptable unless it is a well-known type such as Colt or Guildway, where the individual property is considered on its own merits. HSBC's residential criteria include Colt and Guildway homes in a restricted kit-built category at up to 80% LTV, subject to an entirely satisfactory valuer statement on condition and saleability and normal warranty provision for homes under ten years old. SBI UK lists Colt bungalows as unacceptable for buy to let, and Kensington excludes Colt construction from its residential lending (June 2026).

Single skin walls

Single skin walls are one brick or block thick, without a cavity. Lenders that address them usually allow only a limited element: Aldermore considers an element of single skin but not 100% single skin, Coventry Building Society allows single skin only for single-storey garages and porches, or for a small part of the living space subject to the valuer's guidance, and TSB's buy to let criteria allow it only for a ground-floor extension. Together's index lists 65% for single skin brick and block work, subject to valuation and to Together's classification, product and full assessment.

Roofs, access and insulation: flat roofs, deck access, thatch and spray foam

Some features make a home non-standard at one lender and standard at another, so the lender's own list decides.

Flat roofs

Virgin Money treats felt, asphalt, lead, zinc and GRP (glass reinforced plastic, a fibreglass roof covering) flat roofs as standard, and Nationwide lists felt and asphalt among traditional roof coverings. Others restrict them: some decline a majority flat roof, and Metro Bank limits flat roof to 25% on loans above 95% LTV. What the roof is covered with, and how much of it is flat, are the questions to answer. See flat roof mortgages for buy to let.

Deck access

Deck access flats are reached from an open walkway or balcony rather than an internal corridor. NatWest considers deck access subject to the valuer's comments, Metro Bank accepts balcony or deck access subject to the surveyor's comments, and Kensington considers flats with balcony or open-deck access under its standard product terms (in England and Wales, a minimum property value of £200,000 and up to 80% LTV, June 2026). Coventry Building Society will not consider ex-local authority flats with external deck access. Together's index lists 75% for flats with deck access, subject to valuation and to Together's classification, product and full assessment.

Thatch

There is no general loan to value band for thatch. Many lenders accept a thatched roof within their normal criteria, subject to the valuer and to buildings insurance: Nationwide and The Mortgage Works list thatch as traditional construction on normal terms, Virgin Money treats it as standard and Metro Bank accepts it subject to the surveyor's comments. Some lenders decline thatch, including Kensington, and TSB lists thatched properties as unacceptable for buy to let. Together's index lists 65% for a thatched roof, subject to valuation and to Together's classification, product and full assessment. Listed thatched homes are covered in mortgages for listed buildings with thatched roofs.

Spray foam insulation

Spray foam is commonly declined or referred, and the evidence a lender wants depends on the foam's type and where it is. Nationwide will not lend where spray foam is in the wall voids, but will reconsider foam in the rafters, frame or underfloor with evidence that it was installed in line with the manufacturer's recommendations (supported by a suitability report, a condensation risk assessment and BBA certification) and a transferable warranty, or an invasive inspection by a chartered surveyor where the foam was installed more than two years ago. Coventry Building Society considers loft spray foam only if it is breathable, certified (BBA, BRE or Kiwa) and covered by a transferable insurance-backed warranty, and NatWest considers it subject to the valuer's comments. Aldermore declines spray foam in the roof space, Kensington declines spray foam, and Accord Mortgages declines homes where spray foam was used even if it has since been removed. Together's index lists 75% for spray foam under the roof, subject to valuation and to Together's classification, product and full assessment. More detail is in our spray foam insulation mortgages guide.

Modern methods of construction, BOPAS and warranties

Lenders that publish a policy on modern methods of construction (MMC), such as homes built from systems manufactured offsite, typically ask for accreditation, a warranty and evidence of design life. BOPAS, the Buildoffsite Property Assurance Scheme, was developed with the Council of Mortgage Lenders to assure lenders that accredited offsite-manufactured systems will last for more than 60 years, or two mortgage terms. Accreditation does not oblige any lender to lend.

NatWest accepts BOPAS-accredited modern methods of construction with an acceptable structural warranty. Nationwide asks about the warranty and BOPAS accreditation, and Vernon Building Society requires BOPAS accreditation for non-standard construction. Metro Bank needs a building warranty, buildings insurance on normal terms, a 60-year design life confirmed by a body such as BRE or BOPAS, and saleability confirmed by its surveyor (July 2026). For a newly built home, see new build mortgages; for building your own, self-build mortgages. Barn conversions with timber cladding or steel frames are covered in mortgages for wood-clad and non-standard construction barn conversions.

Residential and buy to let: where the rules differ

A lender's buy to let rules for a construction type can differ from its residential rules, but buy to let is not always stricter. TSB's buy to let criteria list thatched properties as unacceptable, while its residential property criteria do not mention thatch. Across lenders the difference can be sharper: SBI UK's buy to let criteria list Colt bungalows and BISF as unacceptable, while some lenders consider those types for residential lending (see the BISF and Colt sections above).

Other lenders publish the same construction rules for both. Together's 65% maximum for property it classes as non-standard appears in its regulated first charge, consumer buy to let and unregulated buy to let ranges, and its guides say a full assessment can reduce it. Virgin Money's repaired-PRC conditions appear in both its residential and buy to let criteria. The Mortgage Works, Nationwide's buy to let lender, treats thatch and masonry-clad timber frame built in 1970 or later as traditional construction on normal terms, as Nationwide does. The government-backed 95% route below covers regulated residential mortgages only, not buy to let loans.

For landlords, see the PRC construction mortgage guide; for expats, expat mortgages on non-standard construction; and for Scotland, non-standard construction buy to let in Scotland.

The 2025 Mortgage Guarantee Scheme and non-standard homes

The government's 2025 Mortgage Guarantee Scheme does not itself exclude any construction type; each participating lender applies its own property criteria, which may rule out some non-standard construction. HM Treasury describes the scheme as permanently available from July 2025 to sustain 91% to 95% loan to value mortgages, so that eligible buyers can purchase with a deposit as small as 5% (2025 Mortgage Guarantee Scheme). The published scheme rules contain no construction-type exclusion and no purchase price cap. Under rule 6.1 a participating lender acts under its own ordinary-course lending policies, so its property criteria still decide whether a home is acceptable.

Three rules matter for a non-standard home. A scheme loan must have a loan to value above 90% and no more than 95% (rule 7.1). It must be a regulated mortgage that is not a buy to let loan (rule 3.1(i)). And the scheme measures value as the valuation or, if lower, the purchase price, so a cautious valuation on a non-standard home raises the loan to value and can take a 95% case outside the range. See 95% mortgages.

England, Wales and Scotland

Designation of defective housing is a matter of law in England and Wales; Scotland repealed its equivalent regime in 2018. Part XVI of the Housing Act 1985 extends to England and Wales. In Scotland, section 99 of the Housing (Scotland) Act 2014 repealed Part 14 of the Housing (Scotland) Act 1987, which gave assistance to owners of defective housing, from 30 July 2018. The Scottish Government's repeal guidance says any lending is a commercial decision for mortgage lenders. Either side of the border, the lender's own construction rules decide, so a lender's policy for property in Scotland is checked for the specific system.

How a non-standard construction mortgage is arranged

1. Identify the construction: the exact system, variant and build year, from a previous survey, a repair certificate, warranty documents or a structural engineer's report, and for an attached PRC home, whether the neighbouring homes were repaired.
2. Check insurance: confirm buildings insurance is available for the construction, or for a thatched roof, before applying.
3. Match the lender: compare current published criteria on the system, year, repair evidence, outer walls and intended use, residential or buy to let. Where a lender offers a pre-valuation check of the construction, use it.
4. Valuation and application: give the certificates and reports to the valuer, whose view of condition and saleability can change the lender's decision and the loan to value.
5. Offer and completion: the lender decides whether to make an offer after the valuation, and the offer can carry conditions, such as a specialist report, that must be met before the purchase or remortgage completes.

If the valuer raises the construction

A valuer who identifies non-standard construction, or doubts the home will sell, can lead the lender to lower the loan to value, ask for a specialist report or decline. A decline by one lender reflects that lender's rules and does not mean every lender will decline. Before a new application, the exact construction, the valuer's comments and any repair, warranty or structural evidence are compared against other lenders' current criteria. A new lender normally instructs its own valuation. See mortgage declined after valuation and mortgage declined after survey. Japanese knotweed found at a valuation is assessed separately, under the RICS management categories covered on our Japanese knotweed mortgages page.