How to Finance an Extension: Your Options from Cash to Remortgage
- Jun 9
- 5 min read
You've worked out what you want from the extension. The next question is how to pay for it. For most homeowners, a build of £40,000 or more isn't sitting in the current account.
We're a Kettering-based team that designs and builds extensions in Northamptonshire. Here's a practical guide to the main ways homeowners finance an extension in the UK, what each route is good for, and when to bring a professional in.
How to finance a home extension
The main ways to finance a home extension in the UK are cash savings, borrowing against the property (a further advance, remortgage, or second charge mortgage), an unsecured personal or home improvement loan, or specialist refurbishment finance for larger projects. The right route depends on the size of the project, your existing mortgage, and how much equity you have in your home.
Work out the real cost first
Before talking to any lender, broker or finance provider, you need an honest total project cost. Most extension quotes only cover the build itself. They don't include the surrounding professional fees, surveys, or the planning process.
A reasonable rule of thumb: take the builder's quote, add 15% contingency, then add the professional and approval costs separately. That's your real number.
Typical professional costs to factor in:
Architect or designer fees: £2,500 to £15,000 depending on complexity
Structural engineer: £800 to £3,000 for calculations and steel design
Planning application fees: £258 for a typical English householder application
Building Regulations: £400 to £1,500 plus inspection fees
Party wall surveyor: £900 to £2,500 where shared walls are affected
Furnishings and finishings: £15,000 to £40,000 for kitchens, flooring and decoration
There's also a timing angle. You can't apply for most kinds of extension finance until you've got planning permission for an extension in place. That stage usually takes three to four months on its own.
If you're working with an existing structure (looking to convert a conservatory into an extension rather than building fresh, for example) costs and finance requirements can differ. Worth getting clarity on the scope before approaching any lender.
The main ways to finance an extension
There are four broad categories of extension finance in the UK. Each has a different cost profile, time to funds, and suitable project size.
Cash savings
The simplest route, and the only one without interest costs or fees. Suitable for smaller extensions (typically under £30,000) where you don't want to take on debt for the project. Often combined with one of the borrowing routes for larger builds. The downside is that funding the whole extension from savings ties up your emergency fund, so most people don't go that route alone.
Borrowing against the property
Three forms of this exist, all secured against your home:
Further advance. Additional borrowing from your existing mortgage lender, on top of your current mortgage. Usually the simplest route if you're staying with the same lender. The new borrowing sits at the lender's current product rate, which may be higher than your existing fixed rate. Typically 3 to 6 weeks to draw down.
Remortgage to a new lender. Replacing your current mortgage with a new, larger one. Worth comparing against a further advance because the blended rate across the whole borrowing is sometimes lower. Takes 6 to 10 weeks. Early repayment charges may apply on the existing mortgage if you're in a fixed period.
Second charge mortgage. A separate loan secured against the property, sitting behind the main mortgage. Useful when you've got a competitive existing mortgage rate you don't want to disturb (or heavy early repayment charges on it). Available from around £25,000 upwards. Takes 4 to 6 weeks.
Maximum loan-to-value is usually 85% of the current property value across mainstream lenders, though some specialist lenders go higher.
Unsecured personal loans and home improvement finance
Unsecured loans don't take a charge against your property. They're faster to arrange (usually 1 to 2 weeks) but capped on size and priced higher than secured borrowing. Typical maximums sit between £25,000 and £50,000.
Specialist home improvement finance is a similar product, often arranged through a credit broker working with a panel of lenders. As a builder, we offer this route through Phoenix Financial Consultants. Heritage Build Group is an Introducer Appointed Representative (Financial Services Register No. 1053964) of Phoenix, which is authorised and regulated by the Financial Conduct Authority (FRN: 539195). Phoenix is a credit broker, not a lender, and offers finance from its panel of lenders. All finance is subject to status and credit checks.
Specialist refurbishment or bridging finance
For larger projects (typically £100,000 of borrowing or more), or where mainstream routes don't fit, specialist refurbishment finance and bridging loans can work. These are short-term, asset-based products designed for property developers more than homeowners, but a small number of high-value domestic projects do use them. Rates are higher than mainstream lending. Funds usually available in 2 to 4 weeks.
How much can you borrow against your home?
Two limits apply, and whichever is tighter is the one that matters.
Loan-to-value (LTV). Most mainstream lenders cap at 85% of the current property value, not the projected post-works value. Above 85%, rates jump. A small number of specialist lenders will lend against the post-works valuation with a chartered surveyor's projection and a fixed-price builder contract, but this is the exception rather than the rule.
Income affordability. Standard residential mortgage affordability runs at around 4.5 times income. Higher earners can sometimes access 5 to 5.5 times with the right lender. The new total mortgage commitment (existing plus new) has to pass the lender's stress test.
If you're considering a larger project, like a loft extension added to a ground floor build, the combined borrowing can push you against the LTV ceiling. Worth running the numbers before committing to a builder.
When to bring in a finance professional
Anything beyond a small unsecured loan is worth a conversation with a mortgage broker or independent financial adviser. Two reasons:
There are over 90 lenders in the UK residential mortgage market, with widely different criteria. A broker who arranges extension finance regularly knows which lenders will accept which profiles.
The blended cost of borrowing across an existing mortgage and the new borrowing isn't always obvious. The cheapest headline rate isn't always the cheapest overall option.
We're builders, not financial advisers. We can introduce you to Phoenix for finance through their panel of lenders, but for complex borrowing situations (high LTV, complex income, listed property, large project) a regulated mortgage adviser will give you better, FCA-protected advice.
Get a quote for an extension in Northamptonshire
If you're planning a home extension in Kettering or anywhere else in Northamptonshire and want a realistic build cost to take into your finance conversations, speak to our team. Finance is available through Phoenix Financial Consultants if you'd rather spread the cost. All finance subject to status and credit checks.
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