A developer I know closed a $2 million construction loan in the spring, and the first question at the closing table was the one that trips up most first-time borrowers: who pays the interest while nothing is built yet? The answer is usually the loan itself. That is what an interest reserve on a construction loan does, and how it is sized decides whether your project finishes on budget or stalls at 90% complete.
What Is an Interest Reserve on a Construction Loan?
An interest reserve is a line item inside the construction budget that pays the loan’s monthly interest during the build. Instead of writing a check every month, the lender draws the interest from the reserve and adds it to the loan balance.
The FDIC describes it as a mechanism that lets lenders periodically advance loan funds to pay interest charges on the outstanding balance, with that interest capitalized into the loan. In plain terms, you are borrowing money to pay interest on borrowed money.
The reason it exists is simple. A ground-up project has no income until it is finished and sold or leased. Requiring monthly payments from the borrower’s pocket would drain the cash you need for change orders and contingencies.
How Does an Interest Reserve Work Month to Month?
Interest only accrues on money that has been drawn. On day one, that is usually the land or the first draw. By month ten, it is nearly the full loan.
Each month the servicer calculates interest on the outstanding balance, pays it out of the reserve, and adds it to the loan. The reserve shrinks while the balance grows. Because construction draws are staged against inspections, the balance climbs in steps rather than all at once. If you want the mechanics of those steps, see our guide to the construction draw schedule.
- Early months: small balance, small interest charge.
- Middle months: balance builds with framing, MEP rough-ins, and drywall draws.
- Final months: nearly the full balance is accruing, so the monthly interest peaks.
How Do You Calculate the Interest Reserve?
The standard formula has four inputs: loan amount, average percent outstanding, interest rate, and construction term in months.
Interest reserve = loan amount × average % outstanding × annual rate ÷ 12 × months.
Most lenders start with an assumed average outstanding balance of about 50% because draws ramp up over the term. Some underwrite 60% or more on loans with heavy front-loaded costs, like site work or a large land takedown.
Here is a worked example on a $2,000,000 loan at 10% over a 12-month build:
| Average % outstanding | Interest reserve | Share of loan |
|---|---|---|
| 40% | $80,000 | 4.0% |
| 50% | $100,000 | 5.0% |
| 60% | $120,000 | 6.0% |
| 100% (worst case, full balance) | $200,000 | 10.0% |
Rates on construction debt move with the market. The Federal Reserve raised the federal funds target range to 3.75% to 4.00% on September 16, 2026, and many construction loans float off prime. If your rate is floating, ask your lender whether the reserve is sized at today’s rate or with a cushion above it.
What Happens If the Interest Reserve Runs Out?
This is where deals get hurt. If the build runs long, the reserve is spent while the balance sits at its highest. Interest keeps accruing, and someone has to pay it.
On the same $2 million loan, two extra months at a full balance costs about $33,000 (that is $2,000,000 × 10% ÷ 12 × 2). Your options at that point are all uncomfortable:
- Pay the interest out of pocket from the borrower’s cash reserves.
- Ask the lender to reallocate contingency into the reserve, which leaves less cushion for cost overruns.
- Negotiate a loan modification or extension, usually with a fee and a rate bump.
The FDIC’s guidance flags a related lender concern: an interest reserve can keep a troubled loan looking current when the project is actually falling behind. That is why a good lender will not simply size the reserve and walk away. Expect monitoring at each draw.
From a recent deal: I recently structured a $2.4 million loan for a six-townhome project outside Charlotte. The sponsor’s first budget carried a 9-month interest reserve on a 12-month schedule, and I pushed back. Permitting had slipped on his last two projects. We rebuilt the reserve to cover 13 months and moved the extra cost into equity instead of trimming contingency. The project finished in month 12 with about a month of reserve left over, and that cushion is what let him close his takeout without a scramble.
Looking at a ground-up project and not sure how to size the reserve? We structure these deals nationally. Schedule a 15-minute call →
How Much Interest Reserve Should You Budget?
My rule of thumb: size the reserve to the realistic schedule, not the contractor’s best-case schedule. Then add two to three months of cushion for a project without an experienced team, and one to two for a seasoned one.
A few things push the number up:
- Front-loaded budgets. Heavy land or site costs mean a higher average balance from month one.
- Floating rates. A quarter point can add thousands on a seven-figure loan.
- First-time or thin-track-record sponsors. Lenders build in more cushion. If that describes you, read how borrowers without developer experience qualify.
- Lease-up or sell-out periods. Interest during stabilization is a separate problem from interest during construction.
Is the Interest Reserve Part of the Loan or Extra Equity?
It is part of the loan. The reserve counts toward total project cost and the lender’s loan-to-cost limit. On a loan at 80% of cost, a larger reserve raises total cost, and your equity requirement rises with it.
Some lenders fund the reserve at closing into a controlled account. Others simply advance interest through the servicer as each draw is processed. Either way, ask early. It changes your closing costs and your equity check.
Interest Reserve vs. Paying Interest Out of Pocket
Most developers I work with prefer the reserve, because cash stays available for the surprises every job produces. But it is not free. You pay interest on the interest, and it raises your total borrowing.
Paying out of pocket keeps the balance lower and can shave the all-in cost. It only makes sense if you have liquidity well beyond the equity requirement. For a comparison of short-term structures that carry interest differently, see our breakdown of a construction loan versus a bridge loan. For the broader picture, start with our ground-up construction loan guide.
Frequently Asked Questions
How much is a typical interest reserve on a construction loan?
In the example above it is 5% of the loan at a 50% average balance, but it moves with the rate, the build timeline, and how quickly draws ramp up. Use the formula above to check your own numbers.
Do all construction loans have an interest reserve?
No. Some lenders require monthly interest payments from the borrower instead, particularly with experienced sponsors who have strong cash flow. The reserve is more common on spec and development projects with no income during the build.
Who pays interest after the reserve is used up?
The borrower. If the reserve is exhausted before completion, you pay from your own funds or negotiate a reallocation or extension with the lender.
Can an interest reserve be increased mid-project?
Sometimes, but only by moving money from another budget line such as contingency, or by adding equity. Lenders rarely raise the total loan amount mid-construction.
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This article is for informational purposes only and does not constitute financial, legal, or tax advice. Loan terms, rates, and availability vary by borrower, property, and market conditions. Consult Willowbrook Capital for scenario-specific guidance.