Ask what determines rent and you will get two honest answers that sound contradictory. A landlord will tell you the market sets rent. A developer will tell you cost sets rent. Both are right, because they are answering different questions, and the gap between the two answers explains why housing is built in some places and years and not in others.
By the end of this lesson you will be able to separate those two questions cleanly: what rent an existing building can charge, and what rent a new building needs. You will see why new supply appears only where the first number meets the second, and how each additional dollar of construction cost turns into a specific number of dollars of monthly rent.
Question one: what rent can a landlord charge?
For an existing building, rent is set by the market. A landlord looks at comparable units nearby, the number of vacant apartments competing for tenants, how quickly listings are leasing, and the incomes of the people looking. The landlord sets an asking rent, and the market answers: units lease quickly or they sit.
When units sit, landlords rarely cut the printed rent first. They offer concessions: a free month, a waived fee, a reduced deposit. The rent a tenant actually pays after concessions, averaged over the lease, is the effective rent, and it can be noticeably lower than the asking rent. Effective rent, not asking rent, is what the building actually earns.
The important point is what is absent from this list. The landlord’s original cost is not on it. A building that was bought or built decades ago at a low cost charges today’s market rent, not a rent based on its old cost. A building bought last year at a high price cannot charge more than the market allows just because its owner paid more. Existing buildings’ rents drift with the market regardless of their historic cost.
Question two: what rent does a new building need?
A building that does not exist yet has no market rent. It has a cost, and it has a set of lenders and investors, described in Lesson 2, who will fund that cost only if the building can pay them back. Lesson 3 showed the arithmetic: start with the annual cost of capital, add operating expenses, allow for vacancy, and divide by the units. The result is what we call the required rent.
Required rent is a term we use for the rent at which the pro forma just closes: the rent below which a project does not attract capital and above which it does. It is not a rent anyone charges. It is a threshold. For our illustrative 100-unit building at a total development cost of $240,000 per unit, the threshold is about $2,150 per month.
Where the two answers meet
Put the two questions together and the rule for new supply is short. New housing gets built where achievable market rent is at or above required rent. Where market rent is below required rent, the site sits, the shortage persists, and rents on existing buildings rise with demand until either rents reach the threshold or costs fall to meet them.
This is why a city can have rising rents and no cranes at the same time. Rents on existing buildings respond to demand. New buildings respond to the gap between demand and cost. If cost has risen faster than rent, the gap stays closed no matter how many people want an apartment. And when new buildings do open, they charge the market rent too; the required rent only decided whether they were built.
How cost turns into rent
Because required rent is derived from cost, every change in cost changes required rent by a predictable amount. Under the publication’s default rent-math assumptions, listed below, the relationship is close to linear: each $10,000 of added cost per unit adds about $68 to required monthly rent. That figure is modeled, not observed, and it moves when interest rates, loan terms, or investor return targets move.
Derived from the default assumptions below: 65% loan-to-cost at 6.5% over 30 years, 35% equity at an 8% cash-on-cash target, 5% vacancy. $1,000,000 of added cost on 100 units produces $81,370 of required annual revenue, or $67.81 per unit per month.
| Total development cost per unit | Total development cost | Required rent per unit per month |
|---|---|---|
| $200,000 | $20,000,000 | $1,882 |
| $240,000 (base case) | $24,000,000 | $2,154 |
| $280,000 | $28,000,000 | $2,425 |
| $320,000 | $32,000,000 | $2,696 |
Read the table from top to bottom. Each $40,000 step in cost per unit adds roughly $271 to required rent, which is four steps of $68 with rounding. A building that costs $320,000 per unit needs about $814 more rent each month than one that costs $200,000 per unit, before a single tenant has been found. The market either pays that or the more expensive building is not built.
Debt service = monthly payment on $15,600,000 at 6.5%, 30 years × 12 = $1,183,231
Equity return = $8,400,000 × 8% = $672,000
Required NOI = $1,183,231 + $672,000 = $1,855,231
Required revenue = ($1,855,231 + $600,000 opex) ÷ 0.95 = $2,584,454
Required rent = $2,584,454 ÷ 100 units ÷ 12 = $2,154 per monthThe other rows in the table repeat this arithmetic with the loan and equity scaled to each total development cost, holding operating expenses at $600,000.
Modeled, not observed
Every rent figure in this lesson is the output of a model, not a reading from a market. We do not know what a specific building in a specific city will lease for; we know what a building with these assumed inputs would need. The value of the model is not its precision but its direction: it shows which costs move rent, by roughly how much, and therefore which decisions by builders, lenders, and governments make new housing more or less likely to appear.
When we say a requirement “adds $68 per month to rent,” we mean it raises the required rent of a new building by that amount under these assumptions. Whether a tenant ever pays it depends on the market. If the market will not pay it, the effect is not a higher rent on that building. It is that the building is not built.
Illustrative 100-unit building; base total development cost $24,000,000 ($240,000 per unit). Alternative cases at $200,000, $280,000, and $320,000 per unit hold every other input constant.
Loan-to-cost 65%; interest rate 6.5%; 30-year amortization (mortgage constant 7.585% of the loan per year).
Equity 35% at an 8% cash-on-cash target.
Vacancy and credit loss 5% of gross potential rent.
Operating expenses $6,000 per unit per year ($600,000), held constant across the cost cases.
The $68 per month figure is the rent impact of $10,000 of cost per unit under these assumptions: $1,000,000 of cost on 100 units gives a loan of $650,000 with debt service of $49,301; equity $350,000 × 8% = $28,000; required NOI $77,301; ÷ 0.95 = $81,370; ÷ 100 ÷ 12 = $67.81.
Every figure in this lesson is modeled from the assumptions above using the shared method on the Methodology page. No market rent data is claimed, and the costs behind any real building should be checked against local sources.
If interest rates fell, the same cost would produce a lower required rent; at 5.5% instead of 6.5%, the base case falls from $2,154 to $2,048.
If investors accepted a lower return, required rent would fall; at a 6% equity target instead of 8%, the base case falls to $2,006.
If market rents rose while cost held, more sites would clear the threshold and more building would follow, which over time is what pushes market rents back toward required rent.
If cost per unit fell, through cheaper land, smaller units, less parking, or faster approvals, required rent would fall by about $68 per $10,000 saved.
Next lesson. Required rent depends heavily on the terms of the loan and the return investors expect. Lesson 5: How Apartment Financing Works opens up the debt side of that equation.