Understanding mortgage affordability checks is one of the most important parts of getting a mortgage in the UK. Whether you’re a first-time buyer or remortgaging, lenders will carry out a mortgage affordability test to determine how much you can borrow, and whether the loan is sustainable long term. In this guide, we explain how mortgage affordability is calculated, what lenders look for, and how you can maximise your borrowing potential.
Quick answer
Mortgage affordability in the UK is calculated based on your:
- Income (including salary, bonus, or other earnings)
- Monthly outgoings and financial commitments
- Credit profile
- Deposit size
- Lender-specific criteria
Most lenders will offer between 4.5x and 5.5x income, depending on your circumstances.
What Are Mortgage Affordability Checks?
Mortgage affordability checks are assessments used by lenders to decide how much you can borrow and whether you can afford the monthly repayments.
During a mortgage affordability check, lenders review:
- Your income and employment
- Your monthly spending and debts
- Your credit history
- Your overall financial stability
This is often done at Decision in Principle (DIP) stage.
How do mortgage lenders calculate affordability?
A common question is “How do mortgage lenders calculate affordability?
The answer is that each lender has their own model, but they all look at similar core factors.
Income
Lenders assess:
- Basic salary
- Bonuses, commission, or overtime
- Self-employed income
- Other income streams
Different lenders treat income differently.
For example:
- Some lenders use 100% of bonus income
- Others cap or average it
- Some require 2–3 years of history
This is why choosing the right lender is critical.
Monthly commitments
Lenders assess your outgoings, including:
- Loans and credit cards
- Car finance
- Childcare costs
- Living expenses
Higher outgoings reduce how much you can borrow.
Credit profile
Your credit history affects:
- Whether you are approved
- Which lenders are available
- How much you can borrow
Even small issues can impact your mortgage affordability outcome.
Deposit size
A larger deposit:
- Reduces risk for lenders
- Improves affordability
- Can increase borrowing potential
Lower loan-to-value (LTV) often means better rates and more flexibility.
Stress testing
Lenders apply a mortgage affordability test to ensure you can still afford payments if rates rise. This is one of the biggest factors in limiting borrowing.
UK Mortgage Affordability Rules
Current UK mortgage affordability rules mean lenders must ensure responsible lending.
In practice:
- Most lenders cap borrowing between 4.5x–5.5x income
- Some lenders offer higher multiples in certain cases
- Stress testing is applied to protect against rate increases
These rules are why affordability can vary significantly between lenders.
How to calculate mortgage affordability
If you’re wondering how to calculate affordability, there are two approaches:
Basic estimate
A simple guide:
- £50,000 income = £225,000–£275,000 borrowing
- £100,000 income = £450,000–£550,000 borrowing
This is only a rough guide.
Real lender calculation
To accurately calculate affordability, you need:
- Full income breakdown
- Credit commitments
- Lender-specific criteria
This is where a broker adds real value.
How banks work out mortgage affordability
Many people ask “How do banks work out mortgage affordability?”
The key point:
Each lender has different rules.
For example:
- Some lenders are more generous with bonus income
- Some are stricter on credit commitments
- Some offer higher income multiples
This is why Santander mortgage affordability, NatWest affordability, and Halifax affordability can all produce different results for the same applicant.
Why lender choice matters
Two borrowers with identical incomes can receive very different results.
This is because:
- Lenders calculate affordability differently
- Income is treated differently
- Risk models vary
Choosing the right lender can increase borrowing by £50,000–£300,000+
What we see in the market
Across London and the South East, we regularly see:
- High earners under-assessed by lenders
- Borrowers limited by incorrect lender choice
- Clients surprised by how much they can borrow
In many cases, the issue is not affordability, it’s lender fit.
Need help checking your mortgage affordability?
If you want to check mortgage affordability properly, it’s important to use real lender criteria, not generic calculators.
We can help you:
- Calculate affordability accurately
- Compare lenders across the market
- Maximise your borrowing potential
- Understand your options clearly
Book a confidential mortgage review with Oportfolio Mortgages and get a clear answer in minutes.
Related guide
If you’re buying in London, see our full breakdown:
FAQ: Mortgage Affordability UK
How do mortgage lenders work out affordability?
Lenders assess income, monthly commitments, and apply stress tests to determine how much you can borrow.
How to calculate mortgage affordability UK?
You can estimate using income multiples, but accurate results require a full lender-based affordability check.
What is a mortgage affordability test?
A mortgage affordability test checks whether you can afford repayments now and if interest rates increase.
How do banks calculate mortgage affordability?
Banks use their own internal models, meaning borrowing amounts vary significantly between lenders.
