Do I Need To Tell My Mortgage Lender About An Extension?

In most cases you do not need your mortgage lender's formal permission to build an extension, but it is sensible to tell them, and there are situations where you genuinely must. Your lender holds a legal charge over your home — a stake in the property that secures the loan — so any major structural change touches their interest. Whether you need to notify or seek consent depends on the size of the works, whether you are borrowing more to fund them, and whether your home is leasehold. Checking first avoids problems later, especially when you come to remortgage or sell.

At a glance

Permission usually needed?: Not for most extensions
Must tell lender if: Borrowing more to fund it
Leasehold: Freeholder consent often required
Always update: Buildings insurance

Key Takeaways

You rarely need lender permission for a standard extension — but notifying them is good practice, and some mortgage terms require it for major structural work.
You must involve your lender if you borrow more — a further advance or remortgage to fund the build means a full application and often a revaluation.
Leasehold owners usually need freeholder consent — extending a leasehold property typically requires the freeholder's, and sometimes the lender's, agreement first.
Update your buildings insurance before work starts — the rebuild cost changes, and insurers need to know about the works and any contractors on site.

When You Actually Have To Tell Your Lender

For a typical single-storey extension funded from savings, most lenders do not require you to ask permission. But your mortgage terms and conditions may include a clause about making significant structural alterations, and there are clear cases where you should notify or seek consent.

You are borrowing more to pay for it

If you fund the extension with a further advance from your existing lender, or by remortgaging, the lender is directly involved. They will assess affordability and usually revalue the property. Check what you could borrow with our mortgage affordability calculator and model repayments with the mortgage calculator.

The work is major or structural

Large extensions, loft conversions with structural changes, or anything altering the footprint may fall under a notification clause in your mortgage conditions. When in doubt, a quick call or letter to your lender protects you.

Your property is leasehold

Leasehold homes almost always need the freeholder's consent to alter the building, and your lender may want to see that consent too. More on this below.

Will The Lender Revalue My Home?

If you are only notifying the lender and not borrowing more, they generally will not revalue — your existing mortgage and rate continue as normal. A well-built extension often adds value, but that uplift only formally matters to the lender in two situations:

When you borrow more. A further advance or remortgage triggers a valuation, and a larger, improved home may support extra lending at a better loan-to-value.
When you remortgage later or sell. At that point the new valuation reflects the completed, properly documented work.

This is why keeping paperwork matters. Building regulations sign-off, planning permission (or a lawful development certificate where permitted development applies) and any warranties should be kept safe, because a future lender's solicitor will want to see them. You can get a rough sense of value with our house value calculator.

Buildings Insurance And Consents

Buildings insurance

This is the step people forget. During building work your home is at greater risk, and once complete its rebuild cost rises. You should:

Tell your buildings insurer before work starts — some policies restrict cover during major works or require notification of contractors on site.
Make sure your contractor carries their own public liability and, where relevant, structural warranty cover.
Update your sum insured to reflect the larger property once finished.

Your lender requires the property to be insured for its full rebuild cost as a condition of the mortgage, so an out-of-date policy could technically breach your terms.

If you own a leasehold flat or house, the lease usually forbids structural alterations without the freeholder's written consent (a "licence to alter"). Extending without it can be a breach of the lease with serious consequences. Your lender may also need to approve the change. Our guides to the disadvantages of buying leasehold property and what a charge on a property is explain the underlying legal picture.

Why It Is Worth Doing Properly

Cutting corners tends to surface at the worst moment — when you sell or remortgage. A buyer's solicitor will ask for building regulations completion certificates and planning documents. Missing paperwork, unconsented leasehold works, or an insurance gap can delay a sale, reduce what a buyer will pay, or complicate a remortgage. Doing it by the book keeps your options open.

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Reviewing your credit report beforehand helps you fix errors and strengthen your application for the extra borrowing.

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Frequently Asked Questions

Do I need mortgage lender permission for a small extension?

Usually not, if you are paying for it yourself and it is a standard extension. However, your mortgage terms may require notification for major structural work, so it is wise to check your conditions or ask the lender.

Will my lender revalue my house after an extension?

Not automatically. They only revalue when you borrow more, remortgage, or sell. If you are simply notifying them of self-funded works, your existing mortgage usually continues unchanged.

What if my property is leasehold?

You will almost certainly need the freeholder's written consent (a licence to alter), and possibly your lender's approval. Extending without consent can breach the lease, so sort this before starting.

Do I have to tell my buildings insurer about an extension?

Yes. Notify your insurer before work begins, as cover can be restricted during building works, and update your rebuild sum insured once finished so you remain fully covered and compliant with your mortgage terms.

General information only, not financial advice. Mortgage terms, lease conditions and insurance requirements vary — check your own documents and take professional advice before starting work.