Guarantor Mortgage Calculator — How Much More Can You Borrow?

Lenders don't just look at your payslip when they decide how much to offer you. If a parent or close relative agrees to back your application, most lenders will add their income into the sum too, then work out your borrowing from that larger, combined figure rather than yours alone. That's the whole mechanic behind a guarantor mortgage, and it's what this calculator is built to show you.

You'll hear this called a few different things depending on who you talk to — a guarantor mortgage, a family assist mortgage, or increasingly a Joint Borrower Sole Proprietor (JBSP) arrangement, which is the version most brokers recommend now. The underlying idea is the same in each case: someone else's income gets factored in, and that usually opens the door to a bigger loan than you'd qualify for on your own.

Rather than just handing you a single number, the calculator puts your solo borrowing next to your boosted figure so the difference is obvious at a glance. Change any of the inputs — your income, theirs, the deposit — and both numbers update together, which makes it easy to see which factor actually moves the needle for your situation.

Most high street lenders work to roughly 4.5 times income as their standard ceiling, though this shifts a bit depending on the lender and your overall credit picture. Take someone earning £28,000 — on their own, that supports a mortgage of around £126,000. Bring in a guarantor earning £40,000, and the lender is now working from a combined £68,000, which pushes the figure up to roughly £306,000. That's not a small tweak; it's more than double what one income alone would get you.

A handful of specialist lenders will go further still — 5 or even 5.5 times combined income — but only when the guarantor's financial position is genuinely strong. Some JBSP lenders, such as Skipton's Income Booster, will even consider up to four incomes on one application (for example, two buyers plus two parents), which can push borrowing further still. Worth knowing: any existing debts, whether yours or the guarantor's, chip away at that headline multiple, so it's worth clearing what you can before applying rather than after. If you want a broader sense of what a lender might offer without the guarantor angle, our mortgage affordability calculator is a good place to start, and the mortgage in principle calculator gives you a feel for how close you are to an actual decision in principle.

In practice, it's almost always a parent, and grandparents come up fairly often too — lenders will consider other close relatives, but the further you move from immediate family, the harder it tends to be to get approved. Don't assume this is a quick signature and done, either. The lender will scrutinise the guarantor's finances nearly as closely as yours, sometimes more so, because they're the one on the hook if things go wrong.

What they're generally looking for: the guarantor owns their own home outright or has meaningful equity in it, their credit record is clean, and their income leaves genuine room to absorb the guaranteed payments on top of their own bills. Age matters more than people expect — a retired guarantor isn't automatically ruled out, but a lot of lenders won't let the mortgage term run far past their expected retirement age, which narrows the field. If your guarantor is younger and still working, you'll simply have more lenders willing to say yes.

This is the part that's easy to skim past when you're focused on getting the keys, so it's worth slowing down on. Agreeing to be a guarantor isn't a formality — it's a legal commitment. If payments stop, the lender can and will chase the guarantor for the money. Depending on how the mortgage is structured, that could mean their savings, or in some cases a charge against their own home.

There's an upside too, which people sometimes miss: because the mortgage often shows up on both credit files, paying on time reliably can actually help the guarantor's credit standing, not just yours. The flip side is just as true if payments slip. And even if nothing ever goes wrong, the guarantee itself follows the guarantor around — it counts as a liability they have to declare the next time they apply for a loan, a card, or a mortgage of their own. None of this is a reason to avoid the arrangement, but both sides should sit down with independent advice before signing anything.

A guarantor arrangement isn't right for every family, and it's not the only lever available. If parents are in a position to simply give money rather than tie themselves to years of ongoing liability, a gifted deposit is often the cleaner option — it's a one-off gift, the lender needs a short letter confirming it's not a loan, and once it's handed over, that's the end of their involvement.

Shared ownership is worth a look too, particularly if family help isn't on the table at all. You buy a slice of the property — typically somewhere between 25% and 75% — and pay rent on the remainder, which lowers the mortgage you need. Our shared ownership mortgage calculator walks through the numbers. Beyond that, saving a larger deposit, using a Lifetime ISA for the government bonus, or paying down existing debt before you apply can each move your borrowing figure without involving anyone else's name. And if you're self-employed, it's worth checking our self employed mortgage calculator first, since lenders assess that income differently to a standard payslip.

Let's walk through a fairly typical scenario. Say you're earning £28,000 and you've managed to save a £20,000 deposit — a solid start, but on your own, once £150 a month of existing commitments is factored in, a lender working to 4.5× income would likely offer you somewhere around £118,000. In a lot of the country, that simply doesn't stretch to the kind of property you're after.

Now bring your parent into the picture. They earn £40,000 and agree to a JBSP mortgage with you. Because the lender is now basing its decision on your combined income, that £118,000 jumps to roughly £298,000 — an extra £180,000 of borrowing power. Add your deposit back in and you're looking at a maximum property price close to £318,000, which changes the entire shortlist of homes you could realistically buy.

The number that matters just as much, though, is the monthly repayment. Spread over 30 years at 5%, that works out to around £1,599 a month — and both of you need to be genuinely confident that's affordable, not just today but if circumstances change down the line. It's worth running your own figures through the calculator above rather than relying on this example, since income, deposit and rate all shift the outcome quite a bit.

Worth reading before you commit: MoneyHelper's guidance on family and guarantor mortgages is a genuinely useful starting point, the government's Mortgage Guarantee Scheme page covers a related but different route onto the ladder, and you can always double-check any adviser you're dealing with on the FCA register before handing over any paperwork. The rest of our tools are on the mortgage calculators homepage if you want to keep planning.