The listing says 9.19% cap rate. By the end of this guide you will be able to take that number apart into eleven lines, replace nine of them with figures you sourced yourself, and arrive at 3.54% -- without disputing a single physical fact about the building. The roof is the same roof. The tenant is the same tenant. Every change is a line the seller's pro-forma either left out entirely or priced at the seller's cost rather than at yours.
The single largest of those, and the one almost nobody re-checks, is the property tax. A very large number of American jurisdictions reassess a property when it sells. In California the trigger is a change in ownership under Proposition 13's acquisition-value system (BOE Assessors' Handbook 401). In Michigan the taxable value is uncapped in the year following a transfer of ownership (Michigan Treasury). In Florida, non-homestead residential property is assessed at just value as of the January 1 following a change of ownership, and only then does the 10% annual cap start running again (Fla. Stat. 193.1554). The seller's tax bill is a fact about how long the seller has owned the house. It is not a fact about the house.
On the property worked through in this guide, that one line moves from $2,100 to $3,577.50 on the day of closing. Capitalized at a 6.5% cap rate, the $1,477.50 difference is $22,730.77 of purchase price -- more than the entire inspection findings, more than the negotiation, more than anything else you are likely to argue about.
This guide is about one property. Not a market, not a strategy, not a portfolio. It is the procedure for taking a single address with a single set of seller-supplied numbers and finding out, before your due diligence period expires, whether those numbers are true. You will finish it able to: audit a pro-forma line by line and say where each replacement figure came from; comp a rent from leases that actually signed instead of asking prices that did not; get the tax, the insurance, and the utilities right for a non-owner-occupied property rather than for the seller's; build a capital reserve from the actual ages of the actual systems; age a roof, a furnace, a condenser, a panel, and a sewer lateral yourself on a twenty-minute walkthrough; convert an inspection finding into a specific dollar ask, structured so your lender will actually allow it; verify a tenant rather than a building; solve backwards from your own required return to the highest price that clears it; and write walk-away rules in advance, because the whole point is deciding before you are emotionally committed.
There are eleven checklists and scripts in here and six reference tables. Those are the working part. The prose exists to tell you which one to use and when.
A note before you start: this is general education about how rental property due diligence works, not investment, tax, legal, or insurance advice, and nothing here is a prediction about any property or any market. Every dollar figure is either arithmetic shown on the page or a cited figure with its source and its methodology named. Rents in the examples are stated assumptions, labeled where they are used, and you are meant to replace them with real comps for your own address. The mortgage rate anchor is Freddie Mac's Primary Mortgage Market Survey for the week of 2026-08-20 -- 6.65% for a 30-year fixed and 5.95% for a 15-year (Freddie Mac PMMS) -- which is an owner-occupied conventional survey rate; investment-property loans price above it, and the 7.25% used in the worked example is a stated assumption standing in for a real quote, not a rate anyone has offered you. Landlord-tenant law is state law and this guide does not state it. Eviction procedure, notice periods, security deposit handling and transfer, late fee limits, lease disclosure requirements, and rent regulation all vary enormously and change; where they matter below, the mechanism is named and you are told to go read your own state's statute or ask a real estate attorney licensed there. CalculatorByState takes no lead-generation fees, no affiliate money, and no referral payments from lenders, insurers, property managers, or brokers.
The seller's pro-forma is not a lie, and that is the problem
Almost every objection a first-time buyer raises about a pro-forma is the wrong objection. The numbers on it are usually defensible. They are just answers to a different question than the one you are asking.
"Market rent" is a forecast, not a receipt. The pro-forma line labeled rent is frequently what the seller believes the unit could get, sometimes after improvements the seller is not making. The rent the current tenant actually pays is a different number, it is knowable, and it is often lower.
"Potential" gross income is the definition of gross scheduled rent with no vacancy. It is not wrong. It is a defined term that means twelve months of full rent with no turnover, no non-payment, and no lease-up gap. Presented without a vacancy line under it, it silently becomes a claim.
The expense figures are the seller's actual expenses, which is exactly why they are useless to you. The seller's tax bill reflects the seller's assessment. The seller's insurance premium may be a homeowners policy on a property the seller once lived in. The seller's maintenance number is what the seller happened to spend, which is a description of the seller's tolerance for deferred work, not of the building's needs.
There is no management fee because the seller manages it. If you also self-manage, the fee is still real -- it is your labor, and zeroing it means your return is paying you for weekends rather than for the property. If you do not self-manage, it is a bill.
There is no capital reserve because a reserve is not an expense a cash-basis owner ever records. Nobody writes a check to "roof, 2038." The roof still gets replaced.
Here is the useful way to hold all five at once. Take the pro-forma exactly as written and put it into the site's own rental analysis calculator with vacancy, management, and capital reserve all set to zero, because that is what the pro-forma says. The calculator will fire three separate warnings at you before you have typed anything of your own -- one for the missing reserve, one for the zero vacancy, one for the zero management fee. That is not the calculator being pedantic. Those three lines are worth $6,836.10 a year on the property below, and they are the three that never appear on a flyer.
The five documents that turn a pro-forma into evidence
You do not argue with a pro-forma. You replace it. These five documents are what you replace it with, and every one of them is obtainable inside a normal due diligence period.
| Document | Who has it | What it settles | What it does not settle |
|---|---|---|---|
| Twelve months of bank statements or the property's ledger | Seller | Whether the rent on the rent roll was actually collected, and when | Whether the rent is at market |
| Every signed lease, with every addendum | Seller | Term, rate, deposit held, pet and utility terms, renewal and escalation clauses | Whether the tenant will honor it |
| A tenant estoppel certificate, signed by the tenant | Tenant | What the tenant believes the terms are, what deposit the tenant believes is held, and whether the tenant claims any side agreement or unresolved repair | Anything the tenant does not know |
| The parcel's actual tax bill and the assessor's record | County | The current assessment, the taxing districts, exemptions in force, and where the county sits in its reappraisal cycle | Your bill after reassessment -- you compute that |
| A bindable insurance quote for the address, non-owner-occupied | Your agent | The real premium, the real deductibles, and whether anyone will write it at all | Nothing else, but it is the line most often wrong by four figures |
Request all five in the same email on the day you go under contract. Four of them are free. The fifth costs a phone call.
What this guide assumes you already know
This is a paid guide, so it does not spend your money re-explaining things the site gives away. Cap rate, cash-on-cash return, debt service coverage ratio, the 1% rule, and the specific damage that omitting vacancy, management, and reserves does to a return are all covered free and in depth, with a full worked example per state. Rental property in Ohio is the clearest demonstration of the omitted-expenses problem in the series -- it shows a property that cash flows $34.78 a month with those three lines missing and loses $342.62 a month with them present. Rental property in Florida does the same job where insurance rather than tax is the deciding expense. If you have not read one of those, read it first; it is free and this guide starts where it stops.
What follows assumes all of it and asks a narrower question: not what the numbers mean, but how you find out whether the numbers are true for one specific house.