Interest During Construction: What You'll Really Pay (With Calculator)

Interest during construction is one of the most underestimated costs in a build. Most people budget for the build contract price but forget that they’ll be paying interest-only for 6–12 months while the home is being built.

Key facts:

• You pay interest only on the balance drawn, not the full loan — but payments grow with each stage
• For a $600,000 loan at 6.5%, total construction interest typically runs $15,000–$20,000 over 9 months
• A 0.5% rate difference saves around $3,000 over a standard build — rate comparison matters
• Build delays directly increase interest costs: a 3-month extension can add $5,000–$8,000
• Most construction loans are variable during the build phase (rates can change mid-build)

How does construction loan interest work?

During construction, you pay interest only on the balance actually drawn, not the full approved loan amount. This is the key feature of construction loans — your payments start small and grow as funds are released at each stage.

At each drawdown, your interest payment increases:

Based on $600,000 construction loan at 6.5% variable. Deposit draw assumed at contract signing.

How much total interest will I pay during construction?

For a $600,000 loan at 6.5%, with a 9-month build (typical for a 4-bed home in Australia), the estimated total construction interest is $15,000–$20,000. This amount is in addition to any rent you’re paying during the build period.

The total depends heavily on how long each stage takes. If your builder completes the frame quickly, you spend less time at the higher Stage 2 balance. If your build runs over — due to weather, material delays, or trade shortages — every additional month adds interest at the full current balance. For a $600,000 loan, each extra month at full draw costs approximately $3,250 in interest.

For a faster build (6 months), the total is typically $10,000–$14,000. For a complex custom build running 12–15 months, interest alone can reach $25,000–$35,000. Use the Interest During Build calculator to model your specific loan amount, rate, and build timeline against the standard HIA drawdown percentages.

What factors drive the total interest cost?

1. Loan amount — Larger builds accumulate more interest as more funds are drawn at each stage.

2. Interest rate — A 0.5% rate difference on a $600K loan saves ~$3,000 over a 9-month build. Comparing lenders before approval is one of the most effective ways to reduce construction interest.

3. Build timeline — Every month your build extends adds another month of interest. A 3-month delay can add $5,000–$8,000 in interest costs alone.

4. Stage timing — If Stage 3 (Lock-Up) happens quickly, you’ll pay less time at the $354K balance level. Slow inspections or trade delays at individual stages compound the cost.

5. Fixed or variable rate? — Most construction loans are variable during the build. Some lenders allow you to lock in a fixed rate to start at completion. If rates are rising, this matters — check with your broker.

How can I reduce my construction interest costs?

Choose a builder with a fast timeline. Volume builders often have tighter schedules than custom builders. Compare typical build timelines when getting quotes — a builder who completes in 8 months vs 12 months saves you approximately $13,000 in interest on a $600,000 loan.

Check your lender’s inspection turnaround. Some lenders take 10–15 business days to process a progress inspection; others do it in 5. Slow inspections add weeks to your build timeline. Ask your broker which lenders have faster inspection processes.

Keep savings in offset. Some lenders allow an offset account to operate during construction — even on funds not yet drawn. Check if your lender offers this; it can meaningfully reduce the effective interest charged on the drawn balance.

Negotiate rate at application. Your construction rate is locked at approval. Getting a better rate at application saves money for the entire build phase — a 0.5% improvement is worth more than $3,000 over a 9-month build.

What is the double-up period and how do I manage it?

During your build, you’re paying:

• Construction loan interest (to the bank)
• Rent or mortgage on your current home

This double-up period is the main financial pressure point of building a new home. Factor both into your budget when planning your land settlement date.

Tips for managing the double-up:

• If you own your current home, consider timing your land settlement to sell before major drawdowns begin
• If renting, look for a shorter-term lease that expires around your build completion date
• Consider whether moving in with family during the build is feasible to eliminate rent costs
• Some builders offer a guaranteed completion timeline — this can justify a slightly higher build price

What happens to repayments after construction completes?

Once the final drawdown is complete and the occupation certificate is issued, your loan converts to principal-and-interest repayments. This is a significant increase in your monthly payment.

At 6.5% over 30 years on a $600,000 loan:

• Construction phase (interest only on full balance): ~$3,250/month
• P&I phase after completion: ~$3,795/month

Plan for this step-up in your budget. Some borrowers choose to make voluntary repayments during construction (on funds already drawn) to reduce the principal before P&I kicks in — check if your lender allows this.

Frequently asked questions

Does construction interest get added to the loan or paid monthly?

Construction interest is typically charged monthly to your construction loan account and must be paid monthly — it is not automatically capitalised (added to the loan balance). However, some lenders offer “interest capitalisation” as an option, where the interest accumulates and is added to the principal, with repayments starting only after completion. This option is available from select lenders but increases the total loan balance. Most borrowers are better served by paying interest monthly and keeping the loan balance down.

Can I use an offset account to reduce construction interest?

Yes, but only if your lender specifically offers offset during construction — not all do. Some lenders allow an offset account linked to the construction loan facility so that savings sit against the drawn balance. Others only activate offset once the loan converts to P&I after completion. Ask your broker to confirm whether offset is available during the construction phase, and whether it applies to the drawn balance, the full approved limit, or only the P&I portion. The Offset Impact calculator lets you model potential savings.

General advice only. Interest amounts are estimates based on standard HIA drawdown percentages. Actual interest depends on your loan balance, rate, drawdown timing and lender policies. Verify with your lender.