Key Points
- A let-to-buy is assessed as a single structure, not two separate decisions
- Your current home must pass buy-to-let stress testing as a rental
- Your new purchase must pass residential affordability at the same time
- Rental income is not assessed the way most people expect
- Your existing mortgage is not always removed from the calculation
- Small changes in structure or timing can change the outcome
Quick Explanation
Most let-to-buy guides focus on the idea: keep your current home, rent it out, and buy somewhere new.
On the surface, it looks like two separate decisions: one property as a rental, the other as a residential purchase.
Neither of those works the way most people expect.
A property “working” as a rental isn’t about whether the rent covers the mortgage. It depends on how lenders stress that income and what they’re actually prepared to count.
On the residential side, affordability isn’t just about your income. It also depends on how the lender treats the fact you still own the original property.
That’s why a case can look fine as a rental, look fine as a residential purchase, and still fail once the two are combined.
This guide focuses on that interaction: where the structure breaks and what changes the outcome.
If you’d rather skip straight to your own numbers, you can run both sides together under lender-style assumptions using our let-to-buy calculator.
The numbers used here are illustrative. Each lender applies its own rules, so the exact figures will vary. The behaviour is what matters.
Why Let-to-Buy Feels Like It Should Work (But Often Fails)
Start with the numbers most people look at.
Current home value: £300,000
Existing mortgage: £180,000
Expected rent: £1,200 per month
Rough interest-only payment at around 5%: ~£750 per month
On the surface, that looks comfortable. The rent covers the mortgage with room to spare.
Now look at the onward purchase.
Target purchase price: £400,000
Deposit: £80,000
Required loan: £320,000
Income appears to support the new mortgage.
So both sides look workable. Nothing obvious looks stretched.
This is where most people assume the move should go through.
What Actually Gets Tested (Buy-to-Let Side)
That £1,200 isn’t being tested against the £750 payment.
Lenders don’t use the real mortgage cost. They apply a higher test rate and require the rent to cover a margin above that.
So the question isn’t:
“Does £1,200 cover £750?”
It’s closer to:
“How much borrowing does £1,200 actually support under the lender’s stress assumptions?”
On typical assumptions, that £1,200 might support something closer to £165,000–£210,000 of borrowing.
The existing mortgage is £180,000.
That’s where the numbers stop lining up with some lenders.
If you want the deeper breakdown, this is how buy-to-let lenders stress test the rent.
What Happens on the Residential Side
On the residential side, you’re already going through a full assessment.
It’s not just about income. Lenders are looking at affordability under stress, your commitments, credit profile, the property, and whether the case fits their criteria.
On top of that, they have to decide how to treat your existing property.
If the rental side is strong enough, some lenders treat it as self-contained and move on.
Others aren’t. They’ll still hold some or all of the existing mortgage against you when they assess what you can borrow.
That’s where the numbers start to shift. What looks affordable on its own can tighten once the existing property is factored in properly.
Why Let-to-Buy Cases Fail (And Why Lenders Decline Them)
What matters is how those two sets of numbers land together.
The rent doesn’t support as much borrowing as it looks like it should once it’s stress tested. At the same time, the residential lender may still hold part of the existing mortgage against you.
Those decisions are made independently, but they hit the same affordability pool.
That’s where things stop lining up. The rent doesn’t fully offset the borrowing, the existing mortgage isn’t fully removed, and the new loan has to fit around whatever is left.
Small changes in either side can shift the result completely. That’s why two cases that look almost identical on paper can produce very different outcomes.
Why Results Change Between Lenders
You’re not just finding a lender for the new purchase. You’re trying to find a combination that works on both sides at the same time.
On the buy-to-let side, you’re already limited to lenders whose rental stress and loan-to-value rules fit the property. On the residential side, you’re working within a different set based on your income, credit profile, and the new purchase.
Those two pools don’t always overlap.
You might find a lender that works for the buy-to-let, but not the residential, or one that works for the residential but won’t accept the let-to-buy structure.
So the question is whether there’s a workable overlap, not just whether each side works on its own.
Test Your Let-to-Buy Setup Before You Apply
Most let-to-buy cases look fine until both sides are assessed together.
This runs the rental side and the onward purchase together, shows where the numbers stop lining up, and flags which part of the move is causing the pressure.
When Let-to-Buy Isn’t the Right Move
A lot of let-to-buy cases only exist because selling didn’t quite work in the first place.
At that point, people default to “keep it and rent it out”, without really asking whether they actually want to be a landlord.
From a lending point of view, the problem isn’t just the numbers. It’s that you’re trying to make two things happen at the same time that don’t naturally line up.
You can force a let-to-buy through if everything fits, but a lot of cases stall because both sides have to pass under one set of constraints at the same time.
Sometimes it’s cleaner to separate the steps. That’s where bridging comes in. It deals with the timing first, then you refinance or restructure once the position is simpler.
Sometimes, the better option is just to sell and move on.
» MORE: Bridging finance explained
When Consent to Let Comes up (And Why It Usually Doesn’t Solve This)
Consent-to-let usually comes up as a shortcut.
Instead of switching the property to a buy-to-let mortgage, you ask your current lender for permission to rent it out while keeping the existing deal in place.
Sometimes that works, but it doesn’t solve the same problem as a let-to-buy.
Most lenders won’t let you raise additional borrowing under consent-to-let, so it doesn’t help if your deposit for the next purchase depends on releasing equity.
It’s also lender-specific. Some will allow it, some won’t, and where it is allowed, it’s often time-limited.
So while it can be useful, it doesn’t remove the need for the residential side to work on its own, and it doesn’t create the same flexibility as a full let-to-buy structure.
See Where Your Let-to-Buy Actually Lands
At this point, it comes down to whether the case works once it’s assessed as a single structure.
This runs both sides together and gives you the outcome.
If it doesn’t line up, it shows where the pressure sits and whether it can be fixed.
Let to Buy Guide FAQs
What is a let-to-buy mortgage?
A let-to-buy is where you keep your current home, rent it out, and take a new mortgage to buy somewhere else to live. The existing property is assessed as a buy-to-let, and the new one is assessed as a residential mortgage at the same time.
Can I do a let-to-buy?
Possibly, but it depends on whether both sides work together under lender rules. Your current home has to pass rental stress testing, and your new purchase has to work once that’s taken into account.
If you want to check that properly, you need to run both sides of the move together.
How much can I borrow on a let-to-buy?
There isn’t a single figure. It depends on how much borrowing the rent supports under stress, how the existing mortgage is treated, and what’s left for the residential side.
If you want to test how much you can borrow on a let-to-buy, the only reliable way to check is to assess the full structure.
Will I be approved for a let-to-buy mortgage?
Approval isn’t based on one number. The current property has to pass buy-to-let stress testing, and the new purchase has to work once that’s taken into account.
In practice, it comes down to whether the structure fits lender criteria on both sides at the same time, not just whether either part works on its own.
Why was my let-to-buy declined?
Usually because the case doesn’t fit cleanly under lender rules when both sides are assessed together.
Common reasons include the rent not supporting the borrowing under stress, the existing mortgage still being counted on the residential side, or there being no lender combination that accepts both parts.
Why does a let-to-buy look affordable but still fail?
Because the numbers most people use aren’t the ones lenders use. Rent is stress tested, not compared to the real payment, and the existing mortgage isn’t always removed on the residential side.
The gap between those assumptions is where most cases break. The quickest way to see it is to run a detailed let-to-buy simulation.
How do lenders assess a let-to-buy differently from a normal mortgage?
A normal mortgage is one assessment. A let-to-buy is two separate assessments that have to work together.
The buy-to-let side is driven by rental stress testing. The residential side is driven by affordability and how the existing property is treated. The outcome depends on how those two line up.
Can a let-to-buy be approved by one lender but declined by another?
Yes, and it’s common because you’re dealing with two sets of criteria at once. Each lender applies its own assumptions to rental income, existing borrowing, and affordability.
The result isn’t just about the numbers. It’s about whether there’s a workable overlap between lenders on both sides.
How do I know what’s stopping my let-to-buy from working?
You need to look at both sides together.
Sometimes the limit comes from the rental side under stress. Sometimes it comes from residential affordability. In many cases, it’s the interaction between the two.
A combined check lets you see what’s actually stopping your let-to-buy from working, instead of trying to isolate each part.
Are let-to-buy calculators accurate?
Most aren’t, because they only model one side.
Some estimate buy-to-let borrowing from rent. Others estimate residential borrowing from income. A let-to-buy only works if both sides line up under lender rules.
That’s why it helps to get a combined let-to-buy result, not separate estimates, so you can see whether the structure holds together.
Can I use equity from my current home as a deposit?
Yes, but it changes the structure. Increasing the borrowing on the existing property can weaken the buy-to-let side under stress, which then feeds through into the residential side.
Do both mortgages have to be with the same lender?
Not always, but both sides still have to work together. Even with different lenders, the case has to fit within both sets of criteria at the same time.
How do I know if my let-to-buy will be approved before I apply?
You need to see how both sides are likely to be assessed together.
Looking at the rental property or the residential purchase in isolation won’t give you a reliable answer. What matters is how they interact once lender rules are applied.
Running a combined check helps you check whether the whole let-to-buy structure works before you commit to an application.