A house is worth what similar houses sold for. A building is worth what its income supports. That one difference is why a landlord can add value to a fourplex by signing a lease, and why a rent raise is worth so much more than the rent itself.
What does value add actually mean?
Raising net operating income. Net operating income, or NOI, is the rent a building collects less every cost of running it: vacancy, repairs, money set aside for big replacements, management, taxes, insurance and the common-area bills. It leaves out the mortgage, because the building earns the same NOI whoever owns it and however they paid.
Buyers then price a building by dividing that NOI by a cap rate, the yearly return they want on their money. At a 7% cap rate, a building earning $42,056 a year is worth about $600,800. Every extra dollar of NOI is worth about fourteen dollars of price. That is the whole trick: a $150 rent raise is not worth $150, it is worth what $150 a month of extra income is worth to the next buyer.
Three things move NOI. Rents can go up, costs can come down, or empty units can be filled. Only the first is worked through here, because it is the one a landlord has the most control over and the one sellers most often leave on the table.
The cost side is worth a look all the same. Shopping the insurance, appealing an assessment or putting the common-area lighting on a timer lands in NOI exactly like a rent raise does, and at a 7% cap rate a $400 a year saving is worth about $5,700 of price. It is smaller money than rent, and it arrives without a tenant having to agree to anything.
What does $150 a unit do to the numbers?
Take a fourplex at $499,000 with four units renting at $1,575 a month. The building pays $6,400 a year in property taxes, $3,600 in insurance and $450 a month for common-area water, trash and lighting, and it loses 6% of the rent to vacancy. Management runs 8%, with 5% of rent set aside for maintenance and another 5% for big replacements. The loan in this example is $399,200 over thirty years at 7%.
Then raise every unit by $150, to $1,725, as leases come up.
| At $1,575 | At $1,725 | |
|---|---|---|
| Rent collected | $6,300 | $6,900 |
| Vacancy (6%) | −$378 | −$414 |
| Maintenance (5%) | −$315 | −$345 |
| Big replacements (5%) | −$315 | −$345 |
| Management (8%) | −$504 | −$552 |
| Property taxes | −$533 | −$533 |
| Insurance | −$300 | −$300 |
| Common-area utilities | −$450 | −$450 |
| Mortgage | −$2,656 | −$2,656 |
| Cash flow each month | +$849 | +$1,305 |
These are example figures, not a forecast for any market.
The raise is $600 a month across four units. Cash flow goes up $456 of it. The missing $144 went to the costs that are a share of rent: vacancy, maintenance, replacements and management all grew with the income, which is what makes them worth modelling as percentages rather than fixed amounts.
How much is the raise worth as value?
NOI goes from $42,056 to $47,528 a year, up $5,472. At a 7% cap rate that is the difference between $600,800 and $678,971, so the raise added $78,171 of value.
| NOI a year | Value at a 7% cap rate | Cash flow a month | |
|---|---|---|---|
| $1,575 a unit | $42,056 | $600,800 | $849 |
| $1,650 a unit | $44,792 | $639,885 | $1,077 |
| $1,725 a unit | $47,528 | $678,971 | $1,305 |
Note what the middle row does. Half the raise, $75 a unit, is worth $39,085 of value, almost exactly half of the full one. The relationship is a straight line, so there is no threshold to clear and no point at which the next dollar counts for more. The same fourplex priced at different cap rates shows the other half of this: the cap rate buyers use moves the answer more than the rent does.
What is the rehab allowed to cost?
That $78,171 is the budget for the work that earns the raise. If new kitchens, flooring and a tidier exterior cost $40,000 across four units, the building is worth $38,171 more than it cost you, and the rent is $456 a month better while you own it. At $80,000 the work costs more than the value it creates, and the raise has to come from somewhere cheaper, such as charging separately for a service the building currently absorbs.
Two things usually go wrong with that arithmetic. The first is a raise the market will not pay: $1,725 has to be what comparable units in the same area actually rent for, not what the spreadsheet needs. The second is vacancy during the work, which this example does not include. An empty unit costs the whole $1,575 of rent for every month it sits, plus whatever is spent getting it ready, and one empty unit on a fourplex takes more out than most rehab budgets expect.
What vacancy rate should the plan use?
The 6% above is the example’s assumption. For a sanity check, the Census Bureau’s Housing Vacancies and Homeownership survey put the national rental vacancy rate at 7.3% in the second quarter of 2026. Your own submarket matters far more than the national figure, but a plan built on 3% when the country is at 7.3% is worth a second look.
Raising rents also tends to raise turnover, at least for the first cycle, so a plan that lifts rent 9.5% and holds vacancy flat is assuming two things at once.
Where does the raise actually come from?
Look at the gap between what the units rent for and what the same units nearby rent for. If there is no gap, the value add has to come from the cost side or from income the building is not charging for yet: parking, storage, pet rent, or utilities the landlord currently pays. Each of those lands in NOI the same way a rent raise does, and is worth the same fourteen times its annual amount at a 7% cap rate.
Then price the work before you price the building. The offer that makes sense is the one where the rehab, the lost rent while it happens, and the price together stay below what the finished income supports.