When most people estimate how much they can borrow, they start with a simple calculation: annual income multiplied by a figure somewhere between 4.5 and 5.5.

On that basis, two people earning £150,000 should be able to borrow roughly the same amount.

In practice, they can receive very different answers. One may be offered considerably more than they expected. The other may find their borrowing capped well below what their income appears to support.

The difference is rarely the headline figure. It is what makes up that income, how it is evidenced and other factors that impact monthly affordability.

Not all income is treated equally

Consider two applicants.

The first is employed on a basic salary of £150,000, paid monthly, with no bonus.

The second has a basic salary of £90,000 and received a £60,000 bonus last year.

Both earn £150,000. But to many lenders, they look quite different.

Basic salary is guaranteed and paid consistently, so lenders generally accept it in full. Bonuses are discretionary. They can vary from year to year and may not be paid at all. As a result, lenders often use only a proportion of bonus income, or take an average over two or more years.

If a lender accepts only half of the bonus, the second applicant's income for mortgage purposes falls from £150,000 to £120,000 before any other assessment has begun.

The same principle applies to commission, overtime, allowances and share awards. Restricted stock units and other share schemes can form a significant part of total pay, particularly in technology and financial services, but lenders vary widely in whether they accept them and how they calculate them.

Self-employed and director income

The gap can be even wider for business owners.

A company director may pay themselves a modest salary and take the rest as dividends, leaving further profit in the business. Their true earning capacity could be well above £150,000, while their personal tax return shows much less.

Some lenders assess only the salary and dividends actually drawn. Others can consider the director's share of net profit, including profit retained in the company. For a business owner who has chosen to leave money in the business, that difference can transform the borrowing figure.

Sole traders, partners in partnerships and members of limited liability partnerships raise similar questions. Lenders typically want two or more years of accounts or tax returns, and will look at whether income is stable, rising or falling. A recent drop in profit, even for a sound reason, can weigh heavily on the assessment.

Contractors are another example. Some lenders can assess a contractor on their day rate, converted to an annual figure, rather than on the income shown through a limited company. Others rely strictly on accounts.

Gross income is only the starting point

Although lenders often talk in terms of income multiples, affordability is assessed on a much fuller picture.

Lenders look at what the applicant actually has available each month after tax and outgoings. Two people with identical salaries can therefore produce very different results depending on their circumstances.

Commitments such as car finance, personal loans and credit card balances reduce borrowing capacity. So does the cost of supporting children, including childcare and education costs. Student loan repayments, maintenance payments and pension contributions made through salary sacrifice can all affect the income a lender works from.

A single applicant with no dependants and no debts will usually be able to borrow more than someone on the same salary with three children and a car loan, even though both earn £150,000.

The size of the deposit and the loan

Many lenders offer higher income multiples to applicants with larger deposits, as a lower loan to value reduces the lender's risk. Some also offer enhanced multiples to borrowers above certain income levels or in particular professions.

Time and stability

How long someone has been earning their income also matters.

An applicant who has recently started a new role, is still in a probationary period or has recently changed from employment to self-employment may find some lenders cautious, even if the income itself is strong. Others are comfortable lending from day one of a new contract, particularly for professionals moving within the same field.

Income paid in a foreign currency, income from overseas employers and periods of parental leave can also be treated differently depending on the lender.

Each lender has its own model

Perhaps the most important point is that there is no single answer.

Every lender has its own affordability model, its own view on which income it will accept, and its own approach to commitments, dependants and stress testing. Two lenders can look at exactly the same applicant and arrive at borrowing figures tens or even hundreds of thousands of pounds apart.

That is why online calculators can be misleading for anyone whose income is not straightforward. They apply a generic model to a situation that may need a very specific one.

Presenting income properly

For applicants with complex income, the way the case is presented can make a real difference.

That means identifying lenders whose criteria suit the income structure, providing the right evidence from the outset and explaining anything unusual clearly. A bonus history over several years, confirmation of share vesting schedules or an accountant's figures showing retained profit can all help a lender understand the full picture.

It also means timing. Applying after a strong set of accounts is filed, or once a bonus has been paid and evidenced, can change the outcome.

The same income, a different result

Two people earning £150,000 are not necessarily in the same position as far as a mortgage lender is concerned.

What matters is how that income is earned, how reliable it is, what it has to support and which lender is assessing it.

The income may be identical on paper. The borrowing power depends on finding the lender that understands how it is really made up.