A construction project produces no income while it is being built, but its loan charges interest from the first draw. The interest reserve resolves that mismatch: a line item inside the loan budget from which each month's interest is paid, capitalizing the carry until the project can either sell or generate rent. It is the least glamorous line in the sources-and-uses, and it is involved in a remarkable share of construction workouts, almost always because it was sized to a schedule that did not survive contact with reality.
This article explains how the reserve works mechanically, walks the sizing arithmetic in full, and shows exactly how a schedule slip converts a paper line item into a monthly cash call.
What the reserve is, mechanically
At closing, the approved budget includes an interest reserve line alongside land, hard costs, and soft costs. Each month, accrued interest on the drawn balance is funded from that line (the lender effectively advances loan proceeds to pay itself) and the amount is added to the outstanding balance. No cash leaves the sponsor's account while the line lasts.
Three consequences follow. The reserve is not free money; it is borrowed carry that compounds into the balance the exit must eventually repay. The reserve depletes on a curve, slowly at first and faster as the balance grows. And because it is a budget line, it is subject to the same in-balance testing as every other line: if projected interest through completion exceeds what remains in the line, the loan is out of balance and the lender can require a fix.
The three inputs that size it
The draw curve. Interest accrues on drawn dollars, not the commitment. A project that draws slowly and finishes in a rush accrues less total interest than one that front-loads spending, even on identical budgets. Sizing therefore starts with a month-by-month draw projection or, in shorthand, an assumed average outstanding balance across the construction period.
The schedule. Every month of construction is a month of accrual. The honest input is the schedule the permits, trades, and weather will actually allow, not the one in the investor deck.
The rate. Fixed-rate structures make this input simple; floating structures require an assumption about the index path, and prudent sizing stresses it upward. Published series like the Federal Reserve's H.15 make the base environment observable, but nobody's crystal ball earns credit in underwriting.
The sizing arithmetic, in full
Note what the average-balance shorthand hides: the burn is not level. Early months might accrue a fraction of the figure above while only sitework is drawn; late months, with the balance nearly full, accrue multiples of it. That shape is why the most dangerous slippage is late slippage.
How a schedule slip attacks the reserve
A delay in month three is an annoyance. The same delay in month twelve is expensive, because every added month late in the project accrues interest on close to the full balance. Schedule risk and reserve risk are the same risk wearing different clothes.
This is the standard anatomy of an interest reserve shortfall: not a dramatic failure, just a schedule that drifted while the balance grew. It is also why underwriters read the construction schedule as a credit document, not a logistics document.
Monitoring and rebalancing during construction
Competent lenders track the reserve at every draw: interest funded to date, line remaining, and projected interest through the current completion estimate. When the projection exceeds the remainder, the loan is out of balance, and the standard remedies apply: a sponsor deposit, a reallocation from savings elsewhere in the budget, or a restructuring of the facility.
Sponsors should run the same math monthly, independently, for one simple reason: the earlier a shortfall is surfaced, the cheaper the fix. The dashboard is four numbers: reserve remaining, current monthly burn, months to realistic completion, and projected interest through that date. When the fourth number exceeds the first, the conversation has already started whether anyone schedules it or not. A projected gap raised at month six is a budget conversation. The same gap discovered at month thirteen, with the reserve dry and subcontractors mid-scope, is a workout conversation, the territory covered on our lender stopped funding draws page. The interest-reserve estimator makes the projection trivial to update as the schedule moves.
The reserve and the exit
The reserve's endpoint deserves as much design as its size. For-sale projects stop accruing meaningful carry as closings repay the balance, but only if the sales pace holds. Rental projects keep accruing after completion, through lease-up, when the reserve is typically gone; that carry belongs to the next facility, which is why construction-to-bridge structures budget their own carry reserves for the stabilization period.
The recurring planning error is ending the reserve at certificate of occupancy when the loan will actually remain outstanding for months beyond it. Map the reserve to the date the balance is realistically repaid, not the date the building is done, and the arithmetic stays honest.
The reserve inside the wider structure
The interest reserve does not live alone; it interacts with the rest of the loan's architecture in ways worth knowing before documents are signed.
Contingency is not a spare reserve. The two lines answer different risks (contingency absorbs cost surprises, the reserve absorbs time), and loan agreements typically require lender consent to move dollars between them. A budget that quietly plans to raid one line for the other has simply hidden the same shortfall in a different row.
Floating rates need a stress case. On floating-rate structures, the reserve was sized at an assumed index path, and a material move reprices the burn. Some structures pair the loan with rate protection; where they do not, the honest discipline is testing the reserve at higher assumed rates and knowing in advance which party funds the difference.
For-sale product changes the curve twice. Releases from unit closings pay the balance down, which slows the burn late in the project, but only if closings happen on schedule. A sales delay therefore hits the reserve twice: more months of accrual, at a balance that release payments were supposed to be shrinking.
The reserve is a negotiation, not a formula. Funded at closing or built into the commitment; interest paid current from sponsor cash with the reserve as backstop; replenishment obligations if the line drops below projected need. All are negotiated mechanics. Sponsors who understand them shape the structure; sponsors who do not discover them in the loan agreement's definitions section, usually at a bad moment.
What underwriting looks for
A reserve line earns credibility when its assumptions are visible: a draw curve consistent with the construction schedule, a rate assumption with headroom, a period that runs to realistic repayment, and a sponsor who can articulate the out-of-pocket plan if the stress case arrives anyway. Supervisory guidance for regulated construction lenders has long emphasized exactly this discipline: reserves sized and monitored against project reality rather than set once and forgotten.
There is also a soft signal in how a sponsor talks about the line. Treating the reserve as an accounting formality suggests the schedule has not been interrogated; walking an underwriter through the draw curve, the stress case, and the contingent funding plan suggests the opposite. Few budget lines offer such an efficient way to demonstrate operational maturity before the first draw ever funds.
Evoque Commercial sizes and stress-tests the reserve as part of structuring every construction request, because it is the line most likely to determine whether the loan functions in month twelve the way it promised to in the term sheet.
