Buy-to-Let Explained
If you’re looking at a buy to let, the real question isn’t just “how much can I borrow?”
It’s:
- Does the rent actually support the loan under stress testing
- Is the deal capped by rent or by leverage
- Does it only work on certain product types
- Would a different structure change the outcome
Most calculators don’t show you that. They just give you a number.
→ This one breaks it down.
Looking to keep your current home and rent it out instead? Use our let-to-buy calculator instead.
Buy to let calculator
See what this property could support
Get a quick lender-style estimate based on rent, value, structure, and deal type.
What This BTL Calculator Shows That Most Don’t
- Whether rent or LTV is the real constraint
- Whether the deal works on 2-year vs 5-year products
- How tax position and ownership structure change the outcome
- Whether switching between personal and limited company actually helps
- Flags when the issue is structure, not affordability
How Buy to Let Actually Gets Assessed
Buy to let isn’t assessed like a normal mortgage.
It’s a sequence.
If one part fails, the whole deal fails.
1. Everything starts with the rent
Lenders don’t begin with your income.
They begin with:
“What loan does this rent support under stress?”
That’s the entire foundation.
2. Then they apply a stress test
Not the real rate.
A higher one.
That’s where deals break.
This is why a property that “looks fine” can fail immediately.
3. Then they apply a coverage ratio (ICR)
The rent must exceed the stressed interest by a margin.
That margin changes depending on:
- tax position
- ownership structure
- property type
That’s why two borrowers can get different answers on the same deal.
4. Then they cap it with LTV
Even if the rent supports more:
you still can’t exceed the lender’s LTV limit.
So every deal is boxed in by:
- rent support
- leverage
That’s the framework.
Every buy to let deal sits inside those constraints.
What Kind of Problem Are You Dealing With?
Most buy to let deals don’t fail for random reasons.
They fail in patterns.
Some fail because the numbers don’t work.
Some fail because the lender won’t accept the case.
Some only fail under certain assumptions.
The important thing is not just the result.
It’s understanding which type of problem you’re dealing with.
Why Product Type Changes the Outcome
Some buy to let deals:
- work comfortably on 5-year fixed
- fall apart on 2-year fixed
That’s because lenders stress them differently.
Most calculators ignore this completely.
Does Ownership Structure Change the Result?
Sometimes. Sometimes not.
That’s the problem.
This calculator checks both:
- personal
- limited company
and shows whether changing structure actually improves the outcome.
In some cases, it changes nothing.
In others, it’s the difference between working and failing.
Why Two Lenders Give Different Answers
The property hasn’t changed but the interpretation has.
Differences come from:
- stress rates
- ICR requirements
- product assumptions
- structure treatment
That’s why “just try another lender” is sometimes right and sometimes completely wrong.
For a deeper breakdown of rental stress tests, ICR calculations and lender maths, see how buy-to-let stress testing and ICR actually work.
Why Buy to Let Deals Fall Apart
- Expected rent is too low for the stress test
- Borrowing is too high relative to property value
- Shorter fixed products tighten the calculation
- HMO properties face stricter stress rules
- Ownership structure reduces flexibility
- First-time or portfolio status narrows lender options
It usually comes down to one of those.
That’s what the full report breaks down.
Buy to Let vs Let to Buy
Buy to let and let to buy are often confused.
- Buy to let → purchasing or remortgaging an investment property
- Let to buy → keeping your current home and renting it out while buying another
If you are restructuring your current home, use the let to buy calculator instead.
See How Lenders Are Likely to Read Your Case
Most borrowers compare rates before they know whether a lender will actually like their case.
That’s how people waste time with the wrong bank, get weaker offers, or end up with avoidable declines.
The readiness check gives you an early read on how your case is likely to land, where the pressure points are, and whether lender choice needs more care.
- Avoid wrong lenders
- Spot pressure points
- Understand case fit
- Check before applying
See How Lenders Are Likely to Read Your Case
Mortgage Readiness Check
See how lenders will read your case.
Whether the income pattern looks stable enough to rely on, and how much of it they are prepared to include.
Buy-to-Let FAQ’s
How much can I borrow on a buy to let mortgage?
It depends mainly on the rental income, not just your salary. Lenders calculate borrowing by stress testing the expected rent, which means the same property can support different loan amounts depending on the lender and product.
What rental income do I need for a buy to let?
There isn’t a fixed number. Lenders work backwards from the loan, applying a stress rate and a coverage ratio to determine how much rent is required. That’s why small changes in rent or rate assumptions can significantly change the result.
What is the 125% rule in buy to let?
It refers to the interest coverage ratio (ICR). Lenders typically require the rental income to cover at least 125% of the mortgage interest calculated at a stressed rate, not the actual deal rate.
Why does a buy to let fail affordability?
Usually because the rent doesn’t support the loan once stress tested. In some cases, the deal is instead capped by loan-to-value limits or stricter rules on certain property types or borrower profiles.
Can I use equity from my current home as a deposit?
Yes, but this is where many let to buy cases become unstable.
Releasing equity increases the borrowing on your current property, which can make it harder to pass rental stress testing. The structure has to work on both sides after the equity is released.
Do lenders use my salary for buy to let?
Sometimes, but it’s not the main driver. The rental income is usually the primary factor, with your personal income acting as a secondary check for overall suitability.
Is buy to let based on interest only or repayment?
Most affordability calculations are based on interest-only assumptions, even if you choose a repayment mortgage. This is because lenders assess whether the rent can comfortably cover the interest under stress.
Does ownership structure affect buy to let borrowing?
Sometimes, particularly if you are a higher-rate tax payer on personal income.
Personal and limited company applications can be assessed differently, particularly in how stress rates and coverage ratios are applied. In some cases it changes nothing, in others it changes the outcome completely.
Why do different lenders give different answers on the same deal?
Because each lender uses different assumptions around stress rates, coverage ratios, product types, and structure. The property hasn’t changed, but the interpretation has.
What limits buy to let borrowing the most?
Usually one of two things: the rent doesn’t support the loan under stress testing, or the loan is too high relative to the property value. Everything else tends to be secondary.
Does this calculator tell me if I will be approved?
No. It shows whether the structure could work based on typical lending assumptions. It does not use live lender criteria and is not a lending decision.
