How To Work Out A Safe Borrowing Limit For Your First Home

As a first‑home buyer, the real question isn’t “How much will a bank lend me?” – it’s “How much can I safely borrow and still sleep at night?”

A safe borrowing limit is the home loan size where, even if rates rise by about 3 percentage points and your costs jump, you can still pay the mortgage comfortably, keep a cash buffer and live a normal life. That number is often lower than a bank’s maximum approval, especially under APRA’s 3% serviceability buffer and rising living costs.

In this guide, we’ll walk through how lenders size your borrowing power, then layer on your own safety rules so you end up with a decision‑grade number you can act on this week.

1. Bank borrowing power vs your safe limit

Before you set a budget, you need to separate two very different numbers:

1. Maximum bank borrowing power – what a lender might approve under their rules.
2. Your safe borrowing limit – a lower number that fits your lifestyle, goals and risk tolerance.

1.1 How lenders work out your maximum

Most Australian lenders use a similar framework, guided by APRA and the National Consumer Credit Protection Act:

• Gross income – salary, wages, some overtime and bonuses, some government payments.
• Adjustments/shading – they usually shade overtime, bonus, commission and rent (often using 60–80% of the actual amount).
• Living expenses – they take the higher of what you declare or the HEM benchmark (Household Expenditure Measure).
• Other debts – credit cards (assessed at 3–4% of the limit per month), personal loans, HECS/HELP, buy now pay later.
• Assessment rate – they test your repayments at the actual rate plus at least 3% (APRA guidance; most lenders follow this [9]).

From there, they calculate the largest repayment you can “afford” on paper, then convert that into a maximum loan amount over, usually, 30 years on principal and interest.

1.2 Why the bank’s maximum is often too high

Regulators and lenders do try to prevent obvious mortgage stress, but:

• HEM benchmarks can underestimate real costs (childcare, private health, sport, travel).
• Banks don’t see your lifestyle goals – travel, kids, self‑education, starting a business.
• They don’t build in a proper cash buffer for emergencies.

For many professionals and self‑employed borrowers, a practical ceiling is keeping total home (and investment) loan repayments under 30–35% of after‑tax income when tested at rates 3% above current levels [10–12, 15–17]. That’s a useful rule of thumb for first‑home buyers as well.

Quick answer: aim to borrow less than the bank’s maximum so that at current rate + 3%, your repayments are no more than about 30–35% of your take‑home pay, and you can still keep a cash buffer.

2. The two tests every first‑home buyer should run

Think of borrowing power as passing two separate tests:

1. Bank serviceability test – do you meet the lender’s rules?
2. Personal safety test – does the loan still work if life and rates get harder?

2.1 Bank serviceability: the non‑negotiable gatekeeper

Most lenders will:

• Use an assessment rate of actual interest rate + 3% (e.g. a 6% loan is tested at ~9%).
• Assume a 30‑year term on principal & interest (P&I) for your home.
• Add in all debts, even if you plan to close them later (unless formally paid out at settlement).

If the numbers don’t fit, you simply won’t be approved. Understanding how APRA buffers, HEM and other settings cap you is covered in more depth in /insights/apra-buffers-hem-rental-shading-next-geared-purchase.

2.2 Personal safety: your own red lines

Your personal rules might be tighter than the bank’s. Common safety rules I encourage:

• Rule 1 – 30–35% after‑tax limit: Model repayments at current rates + 3% and keep total home loan repayments under about 30–35% of your net income [10–12].
• Rule 2 – Cash buffer: Aim to hold 3–6 months of essential living costs plus mortgage repayments in cash or offset. For larger loans later, 6–12 months becomes more important [3, 18].
• Rule 3 – Life goals: Ensure you can still afford planned children, travel, or starting a business.

If a bank would lend you $750,000 but you’d need to sacrifice everything you care about to make it work, then $750,000 is not your safe limit.

3. Worked example: turning income into a safe loan size

Let’s put concrete numbers around this. Assume:

• Buyer: single professional in Sydney
• Gross income: $110,000
• Approximate after‑tax income: $77,000 p.a. ($6,416 per month)
• Current variable rate scenario: assume 6.0% p.a. (illustrative only)
• APRA buffer: +3% → 9.0% assessment rate

3.1 What the bank might lend (illustrative)

Suppose the bank, using its internal calculator, decides you can afford assessment repayments of $3,600 per month.

At 9.0% over 30 years, repayments of ~$3,600 per month correspond to a loan of roughly $480,000–$500,000.

On paper, your maximum borrowing power is around $490,000.

3.2 Applying your 30–35% safety rule

Using our rule:

• 30% of after‑tax income: 0.30 × $6,416 ≈ $1,925/month
• 35% of after‑tax income: 0.35 × $6,416 ≈ $2,245/month

Now apply that to a current rate + 3% stress test:

• Current rate (illustrative): 6.0%
• Stress test rate: 9.0%

At 9.0% over 30 years:

• A $350,000 loan → roughly $2,820/month
• A $300,000 loan → roughly $2,420/month

Both are above your 30–35% band. To get repayments down to the $1,925–$2,245 range at 9.0%, we’re closer to:

• Loan size around $240,000–$280,000.

That’s a huge gap between what the bank might approve ($490,000) and what fits your safety rules ($260,000 midpoint). In reality, we’d refine this with your exact tax, expenses and deposit.

3.3 Why we still use both numbers

• Bank max tells you the absolute upper bound and which lenders might work.
• Safety cap tells you where you’re still comfortable if rates rise sharply.

Your buying strategy then becomes:

• Target property price = safety‑based borrowing limit + your deposit + any usable schemes.
• Use deposit strategies to close the gap rather than stretching the loan itself – see /insights/deposit-strategies-first-home-buyers-good-broker-guide.