I Want to Extend My Home — How Can I Afford to Fund It?

Most UK homeowners fund an extension in one of five ways: savings, remortgaging to borrow more, a further advance from their current lender, a secured (second-charge) loan, or an unsecured personal loan, with equity release as an option for the over-55s. If you have decent equity and your fixed rate is ending, remortgaging for an extension is usually the cheapest money you can borrow, because it is priced at mortgage rates. If you are mid-deal with a big early repayment charge, a further advance or a second-charge loan normally makes more sense. Before borrowing a penny, cost the full project (build, VAT, fees, contingency) and check that the space you gain is worth more to you than simply moving.

I'm Lisa Hayes, founder of Ready Steady Sell. We're an independent comparison service for people selling their homes, so you might wonder why I'm writing about building an extension. The honest answer is that a large share of the homeowners who contact us have been exactly where you are now: a growing family, a kitchen that's too small, a home office squeezed into the spare room. Before they decided to sell, they looked hard at extending. Some should have extended. Some should have moved years earlier. A few borrowed heavily for a build and then had to sell anyway, which is the most expensive outcome of the lot.

This guide walks through every realistic way to pay for an extension in the UK, how remortgaging for an extension actually works, how much you can borrow, what the monthly figures look like, and how to decide whether to extend or move. I'll show my working with real numbers.

How much does an extension cost in the UK?

You can't choose a funding route until you know how big the bill is. Builders mostly quote per square metre, and the ranges most commonly cited in 2026 are:

Type of extension	Typical cost per m²	Notes
Single-storey rear (average)	£1,800–£3,000	HomeOwners Alliance range, quoted by the Federation of Master Builders
Basic single-storey, flat roof	£1,700–£2,000	Standard windows, simple shape
Standard family specification	£1,900–£2,600	Where most projects land
Premium (structural glazing, complex roof)	£2,800–£3,500	Bifolds and big openings need heavier steelwork
London and the South East	20–50% above the national average	Labour, access and parking all push prices up

The per-metre price is only the start. The Federation of Master Builders (FMB) makes three points that are easy to miss when you're budgeting:

• VAT at 20% applies to labour and materials on an extension. Always ask whether a quote is inclusive or exclusive of VAT, because a £40,000 quote can quietly become £48,000.
• Professional and compliance fees (measured survey, drawings, structural engineer, building control, party wall surveyor) typically add around 15–20% to the budget.
• Small extensions are expensive per metre. Scaffolding, foundations and site set-up don't shrink just because the room does, so a 3m × 3m box can cost disproportionately more per square metre than a 3m × 6m one.

Then there are the costs nobody puts in the first spreadsheet: a temporary kitchen, storage, eating out for six weeks, and sometimes renting elsewhere while the back of your house is open to the weather. Build in a contingency of at least 10%. Nearly everyone who tells me their extension "went over" had no contingency at all.

What are the ways to fund a house extension?

There are six realistic routes. Here they are side by side, before we go through each one properly.

Funding route	Typical size	Cost of borrowing	Secured on your home?	Best for
Savings	Whatever you have	No interest (you lose savings interest)	No	Smaller builds, or topping up borrowing
Remortgage (borrow more)	Large sums	Mortgage rates, usually the lowest	Yes	Fixed rate ending soon, plenty of equity
Further advance	Large sums	Mortgage-type rates; can be higher than switching	Yes	Mid-deal borrowers who want to keep their current rate
Second-charge (secured) loan	Often £10,000+	Higher than a mortgage	Yes	Locked into a cheap fix with a big exit penalty
Unsecured personal loan	Usually up to about £25,000	Higher, over shorter terms	No	Smaller extensions, good credit record
Equity release (lifetime mortgage)	Depends on age and value	Interest rolls up over time	Yes	Homeowners aged 55+ with no plans to move

1. Savings

Cash is the cheapest money you'll ever spend on a build, because there's no interest and no application. The trade-off is that it empties your safety net at precisely the moment you're doing something unpredictable. My advice is simple: if you pay from savings, keep enough back to cover the contingency plus three months of household bills. Builders find things. Old drains, a soft patch of ground, a wall that isn't holding up what everyone thought it was.

2. Remortgaging to fund an extension

This is the route most people search for, and it's usually the cheapest way to borrow tens of thousands of pounds. You replace your current mortgage with a new, larger one (either with your existing lender or a new one), and the extra borrowing is released to you on completion. Because it's priced as a mortgage and spread over a long term, the monthly cost is low compared with any kind of loan.

It works best when:

• Your fixed or tracker deal is ending in the next few months, so there's no early repayment charge (ERC) to pay.
• You have meaningful equity, meaning the gap between what your home is worth and what you owe.
• Your income comfortably passes the new lender's affordability checks at the higher loan amount.

It works badly when you're two years into a five-year fix. ERCs are usually a percentage of the balance, and paying several thousand pounds just to get out of a cheap deal can wipe out the saving. Get your current lender to confirm the exact ERC in writing before you do any maths.

3. A further advance from your current lender

A further advance is extra borrowing from the lender you already have. Your existing mortgage stays exactly where it is, and the new money sits in a separate sub-account, often at a different rate and sometimes over a different term. There's no ERC because you're not leaving anything.

People tend to assume this is automatically the cheapest option. It often isn't. The rate on the new sub-account can be noticeably higher than what a full remortgage elsewhere would give you. A further advance is convenient, but convenience isn't a reason to overpay. Ask your lender for the further-advance rate and compare it with a whole-of-market broker's remortgage quote.

4. A secured (second-charge) loan

A second-charge loan is secured on your home, behind your main mortgage. It lets you keep a cheap first mortgage untouched while borrowing a larger sum than most unsecured lenders will offer. Since 2016, second-charge mortgages have been regulated by the Financial Conduct Authority in much the same way as first mortgages, so you get proper affordability checks and complaint rights.

The rates are higher than a first mortgage, and the loan is secured on your home, so missed payments put the property at risk. They make sense in one situation above all: you're locked into a low fixed rate with a large ERC, and you need more than an unsecured loan will give you.

One thing sellers often find out too late: a second charge must be repaid when you sell. We've written about how that works in practice in our guide to selling a property with a charge on it.

5. An unsecured personal loan

For a smaller build, such as a modest single-storey kitchen extension or a porch, an unsecured personal loan can be the simplest option. Nothing is secured on your home, applications are quick, and the loan is repaid over a shorter term, often five to seven years. Most mainstream lenders cap personal loans at around £25,000. The monthly payment is higher because the term is shorter, but the total interest can be lower than stretching a small sum over 25 years on a mortgage.

6. Equity release (for over-55s)

A lifetime mortgage lets older homeowners release cash from their property with no monthly repayments, as the interest rolls up and is repaid when the home is sold, usually after death or a move into care. It can fund a ground-floor bedroom and wet room that lets someone stay in their home for years longer. But compounding interest eats equity quickly, and it reduces what you leave behind. Only ever take this with regulated advice from a member of the Equity Release Council.

A 0% purchase credit card can also plug a small gap (a few thousand pounds for fittings, say) if you're disciplined enough to clear it before the promotional period ends. It's not a way to fund the build itself.

How does remortgaging for an extension work, step by step?

1. Get proper quotes first. Lenders will ask how much you need and what it's for. Three written builder quotes, plus drawings, make you a far more convincing borrower than "about £50k".
2. Check your ERC and end date. Ask your lender for your redemption figure and the exact date your deal ends. Many lenders let you secure a new deal up to six months ahead.
3. Work out your equity and loan-to-value (LTV). Equity is value minus mortgage balance. LTV is the loan divided by the value. Each 5% LTV band you drop under typically gets you a better rate.
4. Speak to a whole-of-market mortgage broker. They'll compare a remortgage, a further advance from your current lender and, if relevant, a second-charge loan.
5. Valuation and underwriting. The lender values your home as it stands today, not as it will be with the extension. Some lenders will consider the after-works value, but they may hold back part of the money until a valuer confirms the work is complete.
6. Completion and release of funds. Your old mortgage is paid off, the new one starts, and the extra money is paid to you. Keep it in a separate account and pay the builder in stages against completed work, never large sums upfront.

How much can I remortgage for an extension?

Two limits apply, and whichever is lower wins.

The equity limit. Most mainstream lenders will lend up to 85–90% LTV on a remortgage for home improvements, although the best rates sit at 60% LTV and below. If your home is worth £300,000 and you owe £150,000, you're at 50% LTV. Borrowing another £50,000 takes you to £200,000, or 66.7% LTV, which is still comfortable.

The income limit. Lenders assess affordability against your income and outgoings. Many cap borrowing at around 4.5 times household income, and the Bank of England limits how much lending banks can do at or above that multiple. Childcare, car finance and credit card balances all reduce what you can borrow, so clearing a car loan before you apply can make a real difference.

Worked example: funding a £50,000 rear extension

Let's price a 4m × 4m (16m²) single-storey rear extension on a three-bedroom semi, built to a standard specification outside London. These are illustrative figures; your own quotes will differ.

Cost item	Amount	How we got there
Build	£38,400	16m² × £2,400/m² (mid-range standard spec)
Professional fees	£5,760	15% of build (drawings, structural engineer, building control)
Contingency	£3,840	10% of build
Lawful Development Certificate	£274	Half the £548 householder planning fee in England from April 2026
Party wall, surveys, sundries (allowance)	£1,500	Assumption; varies with neighbours
Total to borrow (rounded)	£50,000	Check every quote for VAT

Now look at how different routes compare for the monthly cost of that extra borrowing. The interest rates below are illustrative, chosen to show the shape of the trade-off, not quotes. Real rates depend on your LTV, credit record and the market on the day you apply.

Route (illustrative rate and term)	Amount	Monthly payment	Total interest
Remortgage at 4.5% over 25 years	£50,000	£278	£33,375
Remortgage at 4.5% over 15 years	£50,000	£382	£18,849
Remortgage at 5.5% over 25 years	£50,000	£307	£42,113
Second-charge loan at 8% over 15 years	£50,000	£478	£36,009
Personal loan at 7% over 7 years	£25,000 (typical cap)	£377	£6,695

Two lessons jump out. First, the cheapest monthly payment is not the cheapest loan. Spreading £50,000 over 25 years costs almost £15,000 more in interest than paying it off in 15 years at the same rate. If you can afford the higher payment, ask for a shorter term on the extra borrowing, or overpay. Second, a single percentage point on the rate adds nearly £9,000 over 25 years. Shopping around for the rate matters more than almost anything else you'll do.

Will an extension add value to my home?

Usually, yes. But not always as much as it costs. Nationwide's research, based on its House Price Index data, found that:

Apply that to our worked example. A £300,000 three-bed semi gaining a fourth bedroom might rise by roughly 10%, or about £30,000. The build cost £50,000. On paper, you've spent £20,000 more than you've added. That's fine if you love the house, the street and the school, and you plan to stay for years. It's a poor deal if you're likely to sell within two or three years.

The bigger risk is over-developing for the street. Every road has a ceiling price, roughly what the best houses on it have sold for. Build past that ceiling and buyers simply won't pay for it. Check what the most expensive comparable homes nearby actually sold for (not what they were listed at) using Land Registry sold prices, or our guide to working out what your house is worth, before you commit.

Bedrooms and bathrooms add the most value. Extra reception space adds less. Conservatories, in my experience talking to buyers, can add surprisingly little, and a badly built one can put people off.

Do I need planning permission to extend?

Not always. Many modest extensions in England are covered by permitted development rights, which let you build without a full planning application, within limits. For a single-storey rear extension, the standard limits are up to 3 metres deep on a semi-detached or terraced house and up to 4 metres on a detached house. Larger rear extensions (up to 6m and 8m respectively) can be possible through the prior approval process, where neighbours are consulted. Permitted development doesn't apply to flats and maisonettes, is restricted in conservation areas and on listed buildings, and may have been removed by an earlier planning condition on your home.

Where you do need permission, a householder planning application in England costs £548 from 1 April 2026 (Planning Portal fee schedule). A Lawful Development Certificate costs half that, and I'd strongly recommend one even when permitted development applies. Here's why, from the seller's side of the table.

Why your extension paperwork matters when you eventually sell

Every week we see sales delayed, and occasionally lost, because an extension was built without the right paperwork. A buyer's solicitor will ask for:

• Planning permission, or a Lawful Development Certificate showing permitted development applied.
• A building regulations completion certificate from building control. This is separate from planning, and every extension needs it.
• Party wall agreements or awards if you built on or near a shared wall. See our guide on selling without a party wall agreement.
• Guarantees and certificates for electrics, glazing and any new heating.

Missing documents usually mean indemnity insurance at best and a renegotiated price at worst. Keep a folder, paper or digital, with every certificate from day one. It'll save your future self weeks.

Should I extend or move house?

This is the question behind almost every "how do I fund an extension" search, even when people don't phrase it that way. So let's compare the two properly, using money you don't get back ("dead money") rather than headline prices.

Same family, same £300,000 three-bed semi with a £150,000 mortgage. Option A: extend for £50,000. Option B: sell and buy a £350,000 four-bed nearby.

	Option A: Extend	Option B: Move
Extra borrowing	£50,000	About £50,000 (price gap)
Stamp Duty Land Tax	£0	£7,500 on £350,000 (HMRC rates)
Estate agent fee (assume 1.2% + VAT)	£0	£4,320
Conveyancing, surveys, removals (assumption)	£0	£5,000
Build cost not recovered in value	About £20,000 (using Nationwide's ~10%)	£0
Approximate "dead money"	About £20,000	About £16,800
Disruption	Months of building work at home	Chain risk, viewings, moving day
You end up with	Same location, bigger house	New location, bigger house

The stamp duty figure uses HMRC's residential rates for a home-mover in England: nothing on the first £125,000, 2% on the next £125,000 (£2,500) and 5% on the £100,000 above £250,000 (£5,000). The agent, legal and removal figures are my assumptions, so swap in your own quotes.

What surprises most people is how close the two options are. Moving isn't the reckless, money-burning choice it's often painted as, and extending isn't automatically the sensible one. The tiebreakers are usually non-financial: do you love where you live? Can you live through a build? Is the plot even big enough to give you what you need?

Extending tends to win when you're in a location you'd struggle to buy back into, your plot has room, and you'll stay for at least five to ten years. Moving tends to win when the house can't physically give you what you need, when you'd be over-developing for the street, or when your mortgage lender won't lend enough for a build anyway.

What if I can't get the funding?

Sometimes the answer is no. Your income doesn't support the extra borrowing, your LTV is too high, or there's something on your credit file a lender won't accept. When that happens, you have a few honest options:

• Shrink the project. A 3m single-storey extension under permitted development costs far less than a double-storey one. Could a loft conversion or garage conversion give you the room instead?
• Phase it. Build the shell now and fit the kitchen later.
• Wait and save. Six to twelve months of saving, plus a lower LTV as you pay down your mortgage, can change the numbers.
• Sell and move. If you're borrowing because you've outgrown the house, the answer may be a different house. Porting your mortgage can help; read our explanation of how porting a mortgage works.

When selling fast makes more sense than extending

I'll be straight: for most people reading this, a fast cash sale is not the right answer. If you've got equity, a stable income and a home you love, extend or move on the open market and you'll get full value.

But there's a group of homeowners for whom it genuinely is the answer. They're people who need to move quickly, can't get extension funding, are already stretched, or are in a house that needs so much work that no lender will touch it. For them, certainty often matters more than squeezing out the last few thousand pounds. Across Ready Steady Sell's completed sales in 2025:

The discount is real, and I won't pretend otherwise. On a £300,000 home, 80–92% means offers of roughly £240,000 to £276,000. You save the agent's fee and often the legal costs, but that doesn't close the whole gap. If you're weighing it up, read our breakdown of what selling a house really costs first, then compare several vetted buyers rather than taking the first figure you're given. Our guide to cash house buyers explains how to spot the good ones.

Watch for any company that promises "up to 100% of market value" and then chips the price down days before exchange. It's the oldest trick in the quick-sale industry, and it's why we only work with buyers we've vetted.

Red flags when funding an extension

• Builders offering "finance" on the doorstep. Arrange your own borrowing. Builder-introduced finance can carry high rates, and you lose negotiating power.
• Consolidating everything into the mortgage. Rolling car finance and credit cards into your extension remortgage lowers the monthly figure but can mean paying interest on a car for 25 years.
• Borrowing to the ceiling. If house prices dip, a high LTV can leave you close to negative equity. Our negative equity guide explains why that matters.
• No contingency. If the budget only works when nothing goes wrong, the budget doesn't work.
• Skipping building control. It feels like saving money until you try to sell.

Frequently asked questions

Can I remortgage to build an extension?

Yes. Most UK lenders allow you to remortgage for home improvements, including extensions, provided you have enough equity and pass affordability checks at the higher loan amount. Lenders will want to know what the money is for and may ask for builder quotes.

Is a further advance or a remortgage better for an extension?

A further advance avoids early repayment charges because you stay with your current lender, so it often suits people mid-deal. A remortgage can get you a lower rate if your deal is ending. Compare both before deciding, as a further-advance rate can be higher than what the wider market offers.

How much does it cost to build an extension in the UK?

The HomeOwners Alliance puts single-storey extensions at roughly £1,800–£3,000 per square metre, with London and the South East noticeably higher. Add 20% VAT if quotes exclude it, 15–20% for professional fees and at least 10% contingency.

Can I get a loan for a home extension without remortgaging?

Yes. A second-charge (secured) loan or an unsecured personal loan both leave your existing mortgage untouched. Secured loans allow bigger sums; unsecured loans are usually capped at around £25,000 and repaid over a shorter term.

Does an extension add more value than it costs?

Not always. Nationwide found an extra bedroom typically adds 10–13% to a home's value, and a large bedroom plus bathroom can add up to 24%. On many homes the build costs more than the value added, so extend for the space you need rather than as an investment.

Is it cheaper to extend or move?

It is often closer than people expect. In our worked example on a £300,000 home, extending left about £20,000 of build cost not recovered in value, while moving cost about £16,800 in stamp duty, fees and removals. Location, disruption and how long you'll stay usually decide it.

Do I need planning permission for a single-storey rear extension?

Often not. Permitted development rights in England typically allow single-storey rear extensions of up to 3m on semi-detached and terraced houses and 4m on detached houses. You will still need building regulations approval, and a Lawful Development Certificate makes a future sale smoother.

If you've done the sums and moving looks like the better route, you can see what your home could sell for in a few minutes. Compare offers from vetted buyers with Ready Steady Sell. It's free, there's no obligation, and you'll know your numbers before you speak to a single builder or broker.