Berks & Bucks Finance
Remortgage to Extend
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Written by Harvey Sandhu, specialist mortgage adviser with over 40 years of experience. Berks & Bucks Finance. Updated 2026.
Can I remortgage to extend?
Yes — remortgaging to release equity from your home is one of the most common ways to fund a house extension in the UK. You borrow more than your current mortgage balance, use the difference to pay for the build, and repay it as part of your new mortgage.
The amount you can borrow depends on how much equity you have and whether your income supports the increased borrowing. This page covers how a remortgage to extend works, what lenders look at, and what to sort out before you apply.
Who this guide is for
This guide is for homeowners who want to use the equity in their home to pay for an extension. Whether you are:
- Planning a single or double-storey extension and working out how to fund it
- Deciding between a remortgage, a further advance, or a secured loan
- Mid-way through a fixed rate and unsure whether switching is worth it
- Self-employed and wondering how lenders will assess your income
- Not sure how much you can borrow against your home
Funding an extension is rarely just about the rate. It is about borrowing the right way for your situation — and getting your planning and paperwork in the right order before you apply.
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What You’ll Find on This Page
How does remortgaging to extend work?
When you remortgage to extend, you borrow a larger amount against your existing property than you currently owe. The extra funds, above your existing mortgage balance, are released to you to pay for the build.
Alternatively, if your existing rate is competitive and you do not want to disturb it, you may be able to take a further advance — additional borrowing from your current lender, kept separate from your main mortgage deal. We cover this below.
Worked example
Current property value: £500,000
Current mortgage balance: £250,000
Remortgage to 75% LTV: £375,000
Equity released for extension: £125,000
Final figures depend on your income, affordability, and lender criteria. Your existing balance is repaid as part of the new mortgage.
How much can I borrow to fund a house extension?
Two limits apply. Whichever one bites first is your ceiling.
Loan-to-value (LTV)
Most lenders will lend up to 80–90% of your property’s current value; few lend against increased value after work. Some go to 90% for the right applicant. The higher the LTV, the fewer lenders and the higher the rate.
Affordability
Your income must support the new, larger mortgage payment. Lenders stress-test the total at a rate above the actual deal rate to check it remains affordable if rates rise.
If your income is complex — self-employed, bonus and commission-heavy, or across multiple sources — lender choice matters significantly. Different lenders interpret the same income very differently.
Extension cost ranges — 2026 (approximate)
Single-storey rear extension: £50,000 – £85,000
Side-return extension: £75,000 – £110,000
Loft conversion (dormer): £55,000 – £95,000
Double-storey extension: £130,000 – £250,000
Add 10–15% contingency to any builder quote. Professional fees, planning, structural engineer, and VAT are rarely included.
Do I need planning permission before I can remortgage to extend?
This will depend on the size of the extension. If planning permission is required, lenders will not release funds against a project that does not have planning permission or confirmed permitted development rights in place.
For larger extensions, many lenders also want to see a structural engineer’s report and a fixed-price builder’s contract before issuing a mortgage offer. Coming to a broker without these documents will slow the application or stall it entirely.
Planning and structural reports typically take 12–20 weeks. The mortgage itself takes 4–8 weeks from application. Start the finance conversation once planning is through — not before.
The right sequence
Architect drawings and planning application
Planning permission granted or permitted development confirmed
Structural engineer’s report and calculations
Fixed-price builder’s contract in place
Mortgage application submitted
Should I remortgage to extend or take a further advance?
When you remortgage to extend, this is the first decision to get right. This is a crucial decision in the process — yet most homeowners tend to choose the wrong option.
Remortgage to a new lender
You replace your existing mortgage with a larger one. Usually offers the most competitive rates. The right choice if your current deal is ending or your existing lender cannot lend more.
Further advance from your existing lender
Additional borrowing on top of your existing mortgage, kept as a separate product. Your current rate remains unchanged. Useful if you are mid-fixed rate with early repayment charges — breaking your current deal to remortgage could cost more than the rate saving justifies.
The right answer depends on how much is left on your current fix, the size of any early repayment charge, and how much you need to borrow. We run both scenarios before recommending either route.
Remortgage vs secured loan — which is right for your extension?
A remortgage replaces your existing mortgage with a larger one. A secured loan — sometimes called a second charge mortgage — sits alongside your existing mortgage as a separate loan secured against your home.
Both release equity. The right choice depends on your situation.
If you are mid-fix with significant early repayment charges, a secured loan lets you borrow the additional funds without disturbing your existing rate. The overall cost can still be lower than paying an ERC to remortgage.
Remortgage
Usually lower interest rate
Replaces your existing mortgage entirely
Best when your current deal is ending
Single monthly payment
4–8 weeks to complete
Secured loan
Rate typically higher than main mortgage
Sits alongside your existing mortgage
Best when you are mid-fix with an ERC
Two separate monthly payments
2–4 weeks to complete
What do lenders look at for a remortgage to extend?
Equity and LTV
The more equity you retain after borrowing, the better your rates and lender choice. Most mainstream lenders cap LTV at 80–90% of the current property value — not the projected post-extension value.
Affordability for a remortgage to extend
The new, larger mortgage must pass the lender’s stress test. All existing commitments — loans, credit cards, other mortgages — are assessed alongside the new payment.
Income type and remortgage to extend criteria"
Self-employed applicants are assessed differently from employed applicants, with some lenders still offering lower borrowing to self-employed applicants.
Credit profile
Clean credit gives access to the full lender market. Recent missed payments or defaults narrow options. Adverse credit cases are possible but need specialist lender matching.
Project documents lenders need to remortgage to extend
Many lenders want to see the builder’s fixed-price quote and planning permission before issuing an offer. Having these ready speeds the application significantly.
3 important things to check before you proceed with a remortgage to extend
Your insurer needs to know
Standard building insurance may not cover active construction work. Check with your insurer before work begins — not after the wall comes down.
Will the property be empty during the build?
Most home insurance policies have a vacancy clause — typically 30 to 60 days. If you need to move out during construction, your cover may not be valid. Confirm this with your insurer before you commit to a build programme.
Lenders will want to see paperwork
Some lenders may want to see the builder’s fixed-price quote and planning permission decision notice before they will issue a mortgage offer. Have these ready before you apply.
Can I remortgage to extend if I am self-employed?
Yes, but lender choice matters more than it does for a straightforward employed applicant.
Most lenders require 2 to 3 years of account statements or tax returns; however, some specialist lenders may consider remortgages with 1 year’s accounts.
Limited company directors are assessed on salary plus dividends or on a share of net profits. Sole traders are assessed on net profit.
Not all lenders assess self-employed income similarly, which can greatly impact your borrowing capacity for the extension.
If you’re reading this thinking, “I still can’t tell if switching is worth it”, that’s normal.
Most homeowners feel more confident when they take the time to carefully review all the details — such as fees, timing, and risks — before submitting any application. It makes the process feel much more manageable and clear.
Can I remortgage after the extension is complete?
Yes, you can. If you have the funds to pay for the extension upfront, it can sometimes make sense to complete the work first and remortgage afterwards.
Once the extension is complete, the property is revalued at its improved value. If the extension has added value, the LTV improves, which can give access to better rates and more lenders than were available before the work was done.
Best lenders for remortgaging to extend
There is no single best lender for remortgaging to extend. The question should be who will remortgage in your situation for the best rate.
Lenders such as Natwest, HSBC and Nationwide could help. However, each has different criteria for different properties, self-employment, and required documents, so lender selection is very important.
When remortgage to extend works well
- Strong equity in the property after borrowing
- Income comfortably supports the larger mortgage
- Planning permission and builder’s contract are already in place
- Current mortgage deal is ending or the lender is willing to do a further advance
- A 10–15% contingency has been budgeted on top of the builder’s quote
When it runs into difficulty
A remortgage to extend can stall for a few common reasons:
- Planning permission has not been granted yet
- The builder’s quote has crept significantly since the project was scoped
- Income is complex and the existing lender cannot assess it properly
- The property is mid-fix with a large early repayment charge
- Insurance for the build period has not been arranged
An extension is a big project, and how you fund it matters as much as the build itself. The earlier you talk it through, the smoother the finance runs alongside the work.
We will look at your equity, your income, and your timeline together — and tell you honestly which route works best.
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Let’s talk about your mortgage needs
You don’t need all the answers yet. We can talk through your situation, your plans, and what’s realistically possible before making any decisions.
Frequently Asked Questions About Remortgage To Extend
Can I remortgage my house to build an extension?
Yes. A remortgage to extend lets you borrow more than you currently owe and use the difference to fund the build. The amount available depends on your equity, your income, and lender criteria. We arrange extension remortgages from £40,000 upwards.
Is it cheaper to remortgage or get a home improvement loan for an extension?
It depends on the size of the borrowing and your current mortgage. Adding the cost to your mortgage usually gives a lower interest rate, but over a longer term. A personal home improvement loan is repaid faster but often at a higher rate. We can compare the total cost of each for your situation.
What is the difference between a remortgage and a further advance?
A remortgage replaces your existing mortgage with a larger one at a new rate. A further advance is additional borrowing from your existing lender, kept separate from your current deal. A further advance can be the better option if you are mid-fix with early repayment charges.
How long does a remortgage for an extension take?
The mortgage itself typically takes 4–8 weeks from application to completion. But planning permission and structural reports need to be in place first — that process usually takes 12–20 weeks. Start the finance conversation once planning is through.
Can I borrow against the post-extension value of my home?
Most mainstream lenders lend against your property’s current value, not its projected post-extension value. A small number of specialist lenders will consider post-works valuation with a chartered surveyor’s projection and a fixed-price contract in place.
What if I am self-employed?
Yes, self-employed applicants can remortgage to fund an extension. Most lenders require two to three years of accounts or tax returns. Lender selection matters more than usual — different lenders assess the same self-employed income very differently.
What if my builder's costs go over budget?
This is common. Always add a 10–15% contingency to any builder’s quote before you apply. If costs overrun beyond what was borrowed, a second charge loan can sometimes be used as a top-up — but this needs planning before the original application, not after.
Return to the Remortgage Guide
For a full overview of remortgaging options and other common situations, see the main Remortgage Hub.
All mortgage products are subject to lender criteria, status, and affordability. Rates and product availability are subject to change. This page is for information only and does not constitute mortgage advice. Berks & Bucks Finance is FCA-regulated. Your home may be repossessed if you do not keep up repayments on your mortgage.