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.

Here’s how it works and how to budget accurately.

The mechanics of construction interest

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.

At each drawdown, your interest payment increases:

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

Total interest during construction: a real example

For a $600,000 loan at 6.5%, with a 9-month build (typical for a 4-bed home):

• Estimated total construction interest: $15,000–$20,000
• This is in addition to your rent during the build period

For a faster build (6 months), the total is lower. For a slower or complex build (12–18 months), it’s higher. Use the Interest During Build calculator to model your specific scenario.

What drives 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.

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.

4. Stage timing — If Stage 3 (Lock-Up) happens quickly, you’ll pay less time at the $354K balance level.

5. Are you on a 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.

Minimising 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.

Check your lender’s inspection turnaround. Some lenders take 10–15 business days to process an inspection; others do it in 5. Slow inspections add weeks to your build timeline.

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 reduce interest meaningfully.

Negotiate rate at application. Your construction rate is locked at approval. Getting a better rate at application saves money for the entire build phase.

Interest during construction vs rent

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 financial pressure point of building a new home. Factor both into your budget.

Tips for managing the double-up:

• If you own your current home, consider timing your land settlement to sell existing property before major drawdowns
• 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

After construction: interest converts to P&I

Once the final drawdown is complete and the occupation certificate is issued, your loan converts to principal-and-interest. At this point, your repayments increase significantly — from interest-only on ~$600K (around $3,250/month at 6.5%) to full P&I (around $3,795/month at 6.5% over 30 years).

Plan for this step-up in your budget.

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.