New Build Warranties for Conversions: The Overlooked UK Finance Handbook Requirement That Can Derail a Sale

One of the most overlooked provisions in the UK Finance Mortgage Lenders' Handbook is the requirement for a suitable new build warranty or professional consultant’s certificate where a property has been converted or substantially altered. 

For anyone who has converted, divided or substantially altered a property, it often surfaces after the works are finished — at the point of sale, when a buyer's mortgage lender asks for cover that was never put in place. 

The Handbook — the instructions conveyancers must follow when acting for a mortgage lender — states: 

"If the property has been built or converted within the past ten years, or is to be occupied for the first time, you must ensure that it was built or converted under a scheme acceptable to us." 

Many people associate this requirement only with newly constructed houses and flats. In practice, the requirement can extend much further. 

It's not just new builds: conversions that may need a warranty 

In recent transactions, I have encountered situations where a warranty was required for: 

• A flat converted into two separate flats. 

• Five flats converted back into a single dwellinghouse. 

• A former social club converted into a residential house. 

• A substantially extended residential property. 

• Newly created residential units formed through change of use. 

The common misconception is that because a building already existed, a new build warranty is unnecessary. However, lenders are often concerned with the quality and compliance of the conversion or extensive works rather than simply the age of the original structure.

What counts as an acceptable warranty or certificate?

Lenders generally accept either a structural warranty from an approved warranty provider or a professional consultant's certificate — sometimes called an architect's certificate — issued by a suitably qualified professional who monitored the works as they progressed. 

Which schemes are acceptable varies between lenders. Each lender sets out its own requirements in Part 2 of the Handbook, and some will not accept a professional consultant's certificate at all, or only in limited circumstances. Because the lender that matters is the one a future buyer uses, it is prudent to arrange cover that a broad range of lenders will accept. 

Why a missing warranty can derail a sale 

Where a property falls within the Handbook requirements and lacks an acceptable warranty, a buyer requiring mortgage finance may struggle to obtain a mortgage offer. 

The issue often only comes to light once a transaction is underway, causing: 

• Delays. 

• Additional legal costs. 

• Renegotiations. 

• Lost buyers. 

• In some cases, the collapse of the transaction altogether. 

Retrospective new build warranties are not a simple solution 

It is sometimes possible to obtain a retrospective new build warranty or certificate. 

However, this should not be viewed as an easy fix. 

The process can be: 

• Time-consuming. 

• Expensive. 

• Dependent on the availability of construction records. 

• Subject to intrusive inspections and further investigations. 

Obtaining one after the event is invariably more complicated than arranging the appropriate cover at the outset.

What developers and property owners should consider 

If you are: 

• Converting a building. 

• Creating additional residential units. 

• Changing a property's use to residential. 

• Undertaking substantial structural alterations or extensions. 

Take advice at an early stage on whether a warranty or professional consultant's certificate will be required for future mortgage lending purposes. 

The cost and effort involved in obtaining appropriate cover during the project is often insignificant compared with the difficulties encountered when a future sale is held up because mortgage lenders will not lend. 

A project may be completed and fully compliant from a planning and building regulations perspective, but if the appropriate warranty requirements have not been considered, the property's future marketability can be significantly affected. 

The question should not only be "Can I build it?" but also "Will a mortgage lender lend against it when I come to sell?" 

New build warranties for conversions: frequently asked questions 

Who is responsible for arranging a new build warranty on a converted property — the developer or the seller? 

Responsibility for a new build warranty usually starts with whoever carries out the conversion or building works. Warranty providers generally require a project to be registered before building work begins, so cover is normally arranged by the developer or the owner commissioning the conversion. On a sale, the practical burden falls on the seller: a buyer's conveyancer will ask for the warranty or certificate as part of the standard enquiries and, if it cannot be produced, the seller may be asked to obtain retrospective cover, agree a price reduction or find a buyer who does not need a mortgage. A seller who bought a converted property without a warranty can inherit the problem, even though they did not carry out the works themselves. 

Can I sell a converted property without a new build warranty? 

Yes. There is no legal requirement in England and Wales to have a new build warranty in order to sell a converted property. The requirement comes from mortgage lenders through the UK Finance Mortgage Lenders' Handbook, so it is a buyer's lender, rather than the law, that may stop a sale proceeding. A cash buyer may be willing to go ahead, though often at a lower price, and will face the same question when they come to sell. Title indemnity insurance is not generally accepted by lenders as a substitute for a warranty or professional consultant's certificate. 

Does a new build warranty transfer to a new owner when the property is sold? 

A new build warranty, also known as a structural warranty, is normally attached to the property rather than to the person who first bought it, so the remaining period of cover usually passes to later owners when the property is sold. The same is generally true of a professional consultant's certificate within its term, although the wording of each certificate should be checked. What matters on a sale is that the documents, including any final certificate from the warranty provider, can be produced, so they should be kept safely with the title deeds. 

Do extensions and loft conversions need a new build warranty for a mortgage? 

An ordinary extension or loft conversion added to an existing home will not usually trigger the new build warranty requirement in the UK Finance Mortgage Lenders' Handbook. The position changes where works create a new dwelling, change the use of a building to residential, or are so extensive that a lender treats the property as substantially rebuilt. Each lender takes its own view, so where a project is at the larger end of the scale, advice before work starts is worthwhile. 

Do mortgage lenders still require a new build warranty after ten years? 

Not usually under the UK Finance Mortgage Lenders' Handbook. Its new build warranty requirement applies to properties built or converted within the past ten years, so once that period has passed it will not normally apply in the same way, which is also why most structural warranties run for ten years. A lender may still raise questions about the works through its valuation, and a buyer's conveyancer will still check planning permission and building regulations approval. 

Next reads 

• Conditional Contracts for the Sale of Land: A Guide for Landowners, Developers and Buyers

• A Right of Way Easement Could Stop Your Development Before It Starts

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Disclaimer

This article is for general information only and is not legal advice. Laws and guidance change and outcomes depend on facts. If you need advice on your situation, please contact us. Grant Saw Solicitors LLP is authorised and regulated by the Solicitors Regulation Authority.

Last updated: 26 October 2026