Mortgage Affordability Assumptions: Income, Debt, Deposit and Housing Costs

Use this guide if...

This guide is useful before changing affordability assumptions in the calculator.

• You are estimating a price range Use it when income, debt, deposit and housing costs need to point to a rough property price.
• You want the limits explained It helps separate housing-cost limits, total debt limits and loan-to-value assumptions.
• You are comparing scenarios Use the same income and debt assumptions while testing different rates, deposits and costs.

Main inputs explained

These inputs decide how much payment room exists and how that payment room becomes a price estimate.

• Gross income Affordability estimates usually start with gross monthly income before tax and deductions.
• Existing monthly debts Car loans, student loans, credit cards and other recurring debts reduce room under a total debt limit.
• Housing-cost limit This is the share of gross income used for the mortgage payment plus recurring housing costs.
• Deposit or down payment Cash put toward the purchase increases the estimated property price and lowers loan-to-value.
• Rate and term The mortgage rate and loan term translate monthly payment room into an estimated loan amount.
• Recurring housing costs Property taxes or rates, insurance and HOA/body corporate fees reduce room for the mortgage payment.

Worked example

Suppose a household has 9,000 of gross monthly income, 650 of existing monthly debt, 90,000 available for deposit and 700 of recurring housing costs.

Gross monthly income

9,000

Used as the base for housing-cost and total debt limits.

Existing monthly debts

650

Subtracted from the total debt limit before estimating room for housing.

Housing-cost limit

28%

Allows about 2,520 per month for mortgage and recurring housing costs.

Other housing costs

700 per month

Leaves about 1,820 per month for the mortgage payment in this scenario.

Rate and term

6.5% for 30 years

Turns the payment room into an estimated loan of about 288,000.

Deposit

90,000

The simplified property price estimate is about 378,000 before other purchase costs.

In this simplified scenario, the housing-cost limit is tighter than the total debt limit, so recurring housing costs and the 6.5% rate control the result.

Result interpretation

Read the result as a scenario based on the assumptions entered, not as a decision rule.

What changes the result most

Before using the tool, gather the inputs or assumptions that are most likely to move the result.

• Mortgage rate A higher rate lowers the loan amount supported by the same payment room.
• Existing debts Debt payments can make the total debt limit tighter than the housing-cost limit.
• Recurring housing costs Insurance, taxes, rates and shared-property fees reduce room for the loan payment.
• Deposit and LTV A larger deposit can increase the price estimate, while a maximum LTV can cap it.

Common mistakes

These are common ways an estimate can become cleaner than the real-world scenario.

• Using take-home pay with gross-income ratios If the calculator asks for gross income, using after-tax income can make the estimate too conservative.
• Forgetting non-loan housing costs Property taxes or rates, insurance and shared-property fees can materially reduce payment room.
• Treating the result as approval Affordability calculators do not replace lender checks, credit review or local borrowing rules.
• Ignoring the deposit source Purchase costs, reserves and lender rules can mean not every saved dollar is usable as a deposit.

Related calculators

Use these calculators to check debt ratios, deposits and payment assumptions separately.

Related guides

Use these guides for mortgage payment and deposit assumptions that connect to affordability.

What to try next

Use the next step that matches the question you want to answer.