Approximately 20% of the UK housing stock is of non-standard construction. Some types are perfectly mortgageable. Others are effectively unlendable without specialist intervention. Knowing the difference saves weeks of wasted applications.\n\nWhat Counts as Non-Standard — Anything other than traditional brick or stone walls with a tile or slate roof on concrete or timber foundations. This includes timber frame (the most common non-standard type, generally well-accepted by lenders), steel frame (including BISF and other post-war types), concrete construction (precast reinforced concrete, in-situ concrete, large panel systems), prefabricated and modular builds (modern methods of construction), thatched roofs, flat roofs covering more than 50% of the total roof area, single-skin construction (no cavity), cob and earth walls, straw bale, converted agricultural buildings where original structural elements remain, and properties with extensive underpinning.\n\nTimber Frame — Modern timber frame construction is widely accepted by virtually all mainstream lenders. The Structural Timber Association certifies builders and the construction method is used for a significant proportion of new-build homes, particularly in Scotland. Older timber frame properties may require a specialist survey confirming structural integrity.\n\nConcrete Construction — This is where most problems arise. Post-war prefabricated concrete homes (Airey, Wimpey No-Fines, Cornish Unit, Reema, Unity, Woolaway, Orlit, and others) were built in large numbers by local authorities. Some suffer from concrete carbonation, which weakens the reinforcement steel over time. Properties that have undergone a PRC Homes-approved repair scheme are generally mortgageable. Those without certification are restricted to cash buyers or the very small number of specialist lenders willing to consider them.\n\nModern Methods of Construction — MMC includes modular builds, cross-laminated timber, structural insulated panels, and 3D-printed homes. Lender acceptance is growing but not universal. Properties built by NHBC-registered builders using BOPAS-certified systems (Buildoffsite Property Assurance Scheme, valid for 60 years) are accepted by most mainstream lenders. Without BOPAS or equivalent certification, options narrow significantly.\n\nFlat Roofs — A flat roof covering more than 25% to 50% of the total roof area triggers non-standard classification with many lenders. The concern is lifespan and maintenance cost. Surveyors assess the roof covering type (felt, EPDM rubber, fibreglass, lead) and condition. A recently recovered flat roof in good condition is usually acceptable. An aging felt roof may require replacement before a lender will proceed.\n\nLenders for Non-Standard — Building societies are typically more flexible than banks. Leeds, Nationwide, Yorkshire, Bath, and Ecology building societies all have experience with non-standard properties. Specialist lenders including Aldermore and Kensington consider non-standard on a case-by-case basis. A whole-of-market broker with experience in non-standard construction is essential.\n\nSurveys — A Level 3 RICS Building Survey is strongly recommended for any non-standard property. For concrete construction, a structural engineer's report is often required by the lender. For thatched properties, a specialist thatch survey confirms remaining life and insurance implications.