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Insurance, Warranties & Mortgages for Natural Materials

How unusual materials can affect a project's finance, and how to find out early

4 min readUpdated November 2026

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Why this can stop a project

Lenders, insurers, and warranty providers need confidence that a building will last and hold its value. Materials they rarely see can trigger more questions, a specialist survey, higher premiums, or a refusal. None of that means the material is a bad choice, but it is much easier to deal with before the design is fixed.

Note. This guide is general information, not financial or insurance advice. Criteria differ and change, so get written confirmation from your own lender, insurer, or warranty provider. A specialist mortgage or insurance broker can help.

What counts as non-standard construction?

Lenders commonly treat brick or stone walls under a tile or slate roof as standard. Anything else may be treated as non-standard, although each lender draws the line differently. Modern timber frame with a recognised warranty is widely accepted, while straw bale, cob, and thatch tend to get more scrutiny.

  • Modern timber frame

    Likely reaction
    Often accepted, may need specialist insurance
    What usually helps
    Structural warranty, build records, timber-frame survey where requested

  • CLT and offsite systems

    Likely reaction
    Varies by system
    What usually helps
    Structural warranty, recognised assurance such as BOPAS where it applies

  • Hempcrete

    Likely reaction
    Case by case
    What usually helps
    Early written confirmation, test evidence, experienced designer

  • Straw bale, cob, or earth

    Likely reaction
    Fewer lenders and insurers
    What usually helps
    Specialist lender or broker, detailed records, surveyor familiar with the method

  • Thatch

    Likely reaction
    Usually possible through specialist routes
    What usually helps
    Condition report, fire precautions, and insurance confirmed before exchange

  • Natural insulation in a conventional wall

    Likely reaction
    Usually less of an issue
    What usually helps
    Record the product and installation in case a later surveyor asks

A rough guide only. Individual policies vary.

Mortgages

Self-builders should check before buying land or finalising the design. The lender may want to see the build method and warranty arrangements from the start. Homebuyers should expect a surveyor to report the construction type and possibly ask for a specialist inspection.

  • Tell the lender or broker the exact wall, roof, and insulation build-up.
  • Ask whether the lender needs a structural warranty, and which providers it accepts.
  • Ask whether a professional consultant's certificate would be acceptable instead. It usually gives shorter, narrower protection than a warranty.
  • Ask about future resale. A lender's view affects your next buyer as well as you.
  • Consider lenders that specialise in this area, such as building societies with ecological lending criteria.

Structural warranties and third-party approval

A structural warranty covers defined defects for a set period, commonly around 10 years. Warranty providers set technical standards and may inspect during the build. Talk to them before you specify, as they may ask for extra evidence on unfamiliar products.

Third-party technical approval, such as a BBA certificate, can make that conversation easier. It is not a legal requirement for every product, and many good natural products may not have one. Ask the provider what evidence it would accept instead, such as test reports or engineer's calculations.

Tip. Get acceptance in writing and tied to the specific product. If a contractor later substitutes another one, check that the warranty position has not changed.

Buildings and site insurance

Standard policies ask about walls and roofs, so answer accurately. Getting these details wrong could affect a claim. For an existing home, tell your insurer before major works start, as ordinary cover may be restricted during a renovation.

TopicDetail
Self-build or renovation insuranceCheck that it covers the structure while it is being built, including materials on site.
Combustible materialsAsk whether the insurer has conditions for timber frame during construction.
Moisture-sensitive materialsKeep evidence that they were stored and installed correctly.
After completionPass on the final specification so that the insurer's record matches the building.

Retrofit and existing homes

Retrofit can raise similar questions. Some lenders and surveyors have been wary of certain spray foam installations, which shows how a single product can affect a later sale. Keep a clear record of what was installed, where, and by whom, and check for an insurance-backed guarantee where it is offered.

Work carried out under PAS 2035 should come with a handover pack that includes test certificates, warranties, and maintenance instructions. Keep it with the deeds or property file.

What to keep on file

A good paper trail helps with insurance claims, warranties, and any future sale.

  • Final drawings and specification, including any substitutions
  • Product datasheets, test reports, certificates, and declarations of performance
  • Building control completion certificate
  • Warranty or professional consultant's certificate
  • Installer details and photographs of concealed work
  • Maintenance instructions, such as limewash or render upkeep

Using Matera

When you enquire about a product, ask the supplier whether it has been used on warranted or mortgaged projects and what evidence it can share. Keep the replies together so they are easy to find when your lender or insurer asks.

Sources

Where the facts in this guide come from. Standards and guidance change, and some sources are suppliers or trade sites, so check the original before relying on a detail.

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