Channel 4 documentary highlights Extraordinary Extensions - but which lenders offer mortgages for home extensions and basement digs?

Mortgage Options for Major Basement Conversions and Home Extensions in the UK.

Recently, a Channel 4 documentary on Extraordinary Extensions showcased one of the most substantial property projects filmed — a £10 million-plus basement excavation beneath a London home, revealing the extraordinary lengths (and costs) some homeowners and developers will go to for extra space and value.

That kind of mega-basement has inspired us to write this guide on how to fund basement digs or pay for home extensions with mortgages.

Why Basement Digs Are Popular in the UK

Basement conversion demand has soared in recent years — especially in London and other high-value property markets — because traditional upward extensions or loft conversions aren’t always possible and land is scarce. Digging down can create high-value additional living space, such as:

• Extra bedrooms and bathrooms

• Home offices and gym spaces

• Cinemas, guest suites or rental units

• Garden basements and entertainment zones

These projects can add significant resale value to your property — often more than the cost of the build itself — and turn cramped homes into generous, usable spaces.

What Is a Basement Dig or Home Extension Mortgage?

There isn’t a specific product called a “basement dig mortgage” or "home extension mortgage" but many UK lenders will consider your basement extension as a major home improvement project when you apply for additional funding.

Aaron Strutt, product director at Trinity Financial, says: "Mortgage lenders have different rules when it comes to homeowners either buying a property or remortgaging with the intention of carrying out basement digs or extensions.

"Some will request a detailed cost breakdown and architectural drawings, while others will simply ask whether the property will be habitable during the works. Fixed rates are cheap at the moment, so if you are planning to take out a mortgage for home improvements, it is a good time to do it, although clearly building costs have increased significantly in recent years."

Here are the most common ways homeowners fund basement excavations:

1. Remortgage to Release Equity

If your property has increased in value since purchase, you may be able to remortgage — that is, take out a new mortgage on the property at higher borrowing levels — and release funds to pay for your basement project.

Because this is secured against your home, rates are often much more competitive than development finance or loans. As with any mortgage, lenders will assess your affordability, credit profile, and property value before approval.

At the moment, banks and building societies are offering higher income multiples, up to 6.5 times single or joint salaries. They are also particularly keen to issue more mortgages. 

2. Additional Borrowing or Further Advance

Sometimes you can stay with your current lender but take out a further advance, increasing your existing mortgage to fund basement works. This is often easier than a full remortgage and keeps all borrowing with a single lender, but it is subject to the lender's affordability rules. 

3. Is it worth waiting until your fixed rate finishes and then remortgaging to access Home Improvement funding? 

Yes. It often makes sense to wait until your fixed-rate term ends rather than apply for additional borrowing with your lender. This is for a couple of reasons... Firstly, existing borrowers' further advance rates are often higher than standard fixed rates. Secondly, additional borrowing fixed-rate loans often have different end dates. This means your mortgage may be split into two parts: one for your main balance and the other for the additional borrowing. This makes your mortgage more complex than it needs to be and makes switching lenders harder without incurring an early repayment charge harder. 

4. Home Improvement Loans / Secured Loans

Specialised home improvement loans or second-charge mortgages are another option. These are secured against your home in addition to your main mortgage and are often used for large renovation projects like basement digs. This is not something Trinity Financial can arrange, but we can refer you to some specialists we work with. 

5. Borrowing the Funds to Pay For Imprvovement Works If You're Buying a New Home

It may be possible to buy a property and raise the cash required to carry out an extension or basement dig if you are moving or purchasing a new property. This will depend on the size of your deposit and your mortgage affordability. If you buy a property and extend the property, you could always remortgage once the property is finished or close to being finished to recoup your money. 

Do You Need to Live in the Property During Works?

Probably, yes. Most high street lenders offering the cheapest rates typically require that the property remain habitable while the work is being done — even for basement projects. This matters because mortgage lenders do not want properties to be empty or for homeowners to undertake such significant work without their express permission.

If you plan to move out temporarily while the basement is excavated, some lenders may be more cautious — particularly if the “improvement” transforms the use of the home or creates separate accommodation. It is also worth informing your building's insurance company that you plan to do work on the property. 

Planning, Regulations & Cost Expectations

Basement excavation is one of the most expensive home improvement projects you can undertake — with costs for full dig-downs and underpinning often running into the hundreds of thousands of pounds.

Before you start:

• Check whether you’ll need planning permission, building regulations, or party wall agreements.

• Prepare for structural surveys, engineering reports, and waterproofing requirements and get a full cost breakdown from a few builders.

Why Basement Builds and Extensions Will Continue to Drive Mortgage Enquiries

The spotlight on mega projects like the Channel 4 “£10–12 million basement” documentary helps inspire homeowners to improve and expand their homes rather than move and incur the associated costs. A basement dig or extension typically provides:

• More space without moving

• Better value for money than moving house

• Increased property value through additional square footage

Whether you’re planning a modest cellar conversion or a full basement dig, understanding your funding options — from equity release and remortgaging to further advances and secured homeowner loans — is essential before you start digging.

Lending solutions with Trinity Financial

Are you looking to buy a property or remortgage to raise funds for home improvements and require expert advice? We’re here to help you find a solution – no matter how complex your circumstances. Our expert brokers have extensive experience providing creative solutions to secure mortgages for our clients.

Call Trinity Financial on 020 7016 0790 to secure a fixed or tracker rate mortgage, book a consultation or use our appointment calendar

The information contained within was correct at the time of publication but is subject to change.

Your mortgage is secured on your property. Your property may be repossessed if you do not keep up repayments on your mortgage