The inclusion list is where most of the errors live, and Publication 527 enumerates it. Five categories account for nearly all of them.
Advance rent is any amount received before the period it covers, and the rule overrides the usual timing question: include it "in your rental income in the year you receive it regardless of the period covered or the method of accounting you use." An accrual-method landlord does not get to spread it. A cash-method landlord has no argument for deferring it either.
A payment to cancel a lease is rent. If a tenant pays to get out of the remaining term, that money is rental income in the year received, again regardless of accounting method.
Expenses a tenant pays on the owner's behalf are rental income, and the same amount is deductible if it is a deductible rental expense. The classic case is a tenant who pays a bill the lease makes the owner responsible for and subtracts it from the rent check. The net cash is smaller, and the gross income reported is not.
Property or services received instead of money go in at fair market value. Where services are provided at an agreed or specified price, Publication 527 treats that price as fair market value unless there is evidence to the contrary. A tenant who paints the building in place of two months' rent has produced two months of taxable rental income for the owner, along with a deduction for the painting.
A lease with an option to buy produces rental income while it is a lease. The payments received under the agreement are generally rental income, and if the tenant exercises the option, payments received for the period after the date of sale become part of the selling price rather than rent.
Security deposits sit outside income until something changes them. An owner does not include a deposit in income on receipt if the plan is to return it at the end of the lease. Whatever is kept because the tenant did not live up to the lease is income in the year it is kept. And the characterization rule matters more than the label: "If an amount called a security deposit is to be used as a final payment of rent, it is advance rent. Include it in your income when you receive it."
The self-employment line is drawn by services, not by effort. Rent from real estate is generally not subject to self-employment tax. IRC 1402(a)(1) excludes "rentals from real estate" from net earnings from self-employment "unless such rentals are received in the course of a trade or business as a real estate dealer." A landlord who spends significant time on the property is still receiving rentals from real estate; time spent is not the test.
What moves the income across the line is set out in Treasury Regulation 1.1402(a)-4(c). Its first paragraph confirms the ordinary case: payments for the use or occupancy of entire private residences, or of living quarters in duplex or multiple-housing units, "are generally rentals from real estate." Its second paragraph supplies the exception. Payments for the use or occupancy of rooms or other space "where services are also rendered to the occupant, such as for the use or occupancy of rooms or other quarters in hotels, boarding houses, or apartment houses furnishing hotel services," do not constitute rentals from real estate and are included in net earnings from self-employment. The regulation then states the principle and illustrates it: services count when they are "primarily for his convenience and are other than those usually or customarily rendered in connection with the rental of rooms or other space for occupancy only," so that "the supplying of maid service, for example, constitutes such service; whereas the furnishing of heat and light, the cleaning of public entrances, exits, stairways and lobbies, the collection of trash, and so forth, are not considered as services rendered to the occupant." Heat, light, common-area cleaning and trash collection are the ordinary incidents of renting property out. Daily housekeeping and hotel-style services are something else, which is why short-term renting with substantial guest services can produce a different answer from renting the same property on an annual lease.
Two further regimes reach rental income and neither is decided by the rent itself. The net investment income tax can apply, and the section 199A deduction can be available, but both turn on facts about the activity, notably whether it rises to a trade or business and how the owner participates in it. Each is its own subject and neither is settled by looking at the receipts.