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was the rent set at the inception of the lease at Fair Rental Price over the term of the lease?  if so, and FRP was subsequently to increase substantially, my unauthoritative position is you are charging FRP. if the IRS were to take a different position thousands of landlords would have substantial tax problems.

from IRS PUB 527

Fair rental price. A fair rental price for your
property is generally the amount of rent that a
person who isn’t related to you would be willing
to pay. The rent you charge isn’t a fair rental

price if it is substantially less than the rents
charged for other properties that are similar to
your property in your area. 
Ask yourself the following questions when
comparing another property with yours.
• Is it used for the same purpose?
• Is it approximately the same size?
• Is it in approximately the same condition?
• Does it have similar furnishings?
• Is it in a similar location?  

what isn't mentioned is when and for what period

June 20, 2016
by Johns Prins, Originations Analyst

One of the most important components of managing a residential rental income property is determining the fair market rental value. Once you determine this calculation, it will help you determine the rent you plan to charge your tenants. These rental incomes will become annual cash flows that will become the fundamental basis for the property’s overall value—so needless to say—the figures you decide on are of tremendous importance.

In this article we’ll cover the following topics:

Explanation of Market Rent
How to Search Rates to Find Fair Market Rent Value
Ways to Find Fair Rental Market Value Calculation
Why You Should Start Your Fair Market Rental Value Calculation Now
How does one derive appropriate rental figures which are reasonable while offering the property the opportunity for the highest short-term cash flows as well as value perception in the present and future? The fair market rental value of the property is a number that rental property investors highly seek.

You Must Understand Market Rent
Perhaps it would be valuable to revisit what market rent is. Market rate, in this sense, refers to the rate in which people are willing to pay, the “going rate”—if you will—in a specific market. In a (relatively) free market setting, rental rates—whether it be apartments, homes, what have you—will be dictated by the market and what it can absorb—a market’s carrying capacity.

There’s certainly some ambiguity surrounding these figures, and some factors that may exceed the control of rental income property owners or managers—but with that said, the following components will both help relevant constituents associated with these rental properties determine what the market will bear as it pertains to pricing—as well as several other concepts that will be valuable to know and understand with a rental investment.

Search Rates to Find Fair Market Rent Value In Your Market
One of the most efficient ways to calculate fair market rental rates for your property is to look at the rents charged by similar properties within the market. Comparing the similar, local properties to your own regarding physical home size, bedrooms, and bathrooms, furnished or non-furnished options—among many other price-influencers, will allow you to build a foundation for the kind of rental rates you seek based on your perceived placement in the marketplace. (MY comment - also the lease term must be taken into consideration - rent would likely be different for a 1 year vs a 2 year lease of the same property).

It is also important to investigate residential vacancy rates in your market before searching comparable rates. Published on a quarterly basis, vacancy rates offer constituents an illustrative idea of the proportional occupancy in a particular market or submarket.

to directly answer your questions as to what happens if fair rental isn't being charged

any day rented at less than fair rental is a personal use day.

if personal use days during a tax year are more (1) 14 days or (2) 10% of total rental days during the year then

was the property rented less than 15 days at fair rental during the year?

if yes, then the property is treated as a personal residence - only mortgage interest and taxes are deductible on Schedule A subject to the rules and limitations for a personal residence

nothing is reported on Schedule E - no depreciation allowed

if no, prorate expense between Fair Rental days and Personal Days.

 the personal portion of interest and taxes is such to the rules and limitations for a personal residence

The rental portion of interest and taxes is not limited to rental income. other rental deductions are limited to the remaining income, if any, and are subject to ordering rules . carryovers are allowed

if you sell the property the gain is figured based on sales price net of selling expenses less your tax basis (cost less depreciation) first depreciation must be recaptured. then a partial home sale exclusion may be available. figured as the number of days(months) occupied as your main personal residence in the 5 years before the date of sale to 730 days (24 months) times the applicable exclusion amount.  you say you moved out for a job change. therefore, to qualify for the partial exclusion the new place of employment must be at least 50 mile farther from the property sold than the former place of employment was.