Can rental property taxes and mortgage interest be deducted only when it is offered for rent?

In the first year when you convert a property from personal use to rental, there are three things that get prorated between the SCH E and SCH A.

Mortgage Interest - For total mortgage interest paid during the tax year, the percentage of mortgage interest that is equal to the percentage of the year the property was classified a residential rental real estate is reported/claimed on SCH E. The remainder is a SCH A itemized deduction.

Property Taxes - For total property taxes "actually paid" during the tax year, the percentage of property taxes that is equal to the percentage of the year the property was classified as residential rental real estate is reported/claimed on SCH E. The remainder is a SCH A itemized deduction.

Property Insurance - For insurance, the amount paid that is equal to the percentage of the tax year the property was classified as a rental is claimed/reported on the SCH E. The remainder is not claimed or deducted anywhere else on the tax return. Property taxes paid for the period of time it was your primary residence, 2nd home, vacation home or any other type of "personal use" is not deductible anywhere.

Please read the below to see if that helps clarify other things for you. If it doesn't, just post back.

Rental Property Dates & Numbers That Matter.

Date of Conversion - If this was your primary residence or 2^nd home before, then this date is the day AFTER you moved out, or the date you decided to lease the property – whichever is later.
In Service Date - This is the date a renter "could" have moved in. Usually, this date is the day you put the FOR RENT sign in the front yard.
Number of days Rented - the day count for this starts from the first day a renter was contracted to move in, and/or "could" have moved in. That would be your "in service" date or after if you were asked for that. Vacant periods between renters do not count for actual days rented. Please see IRS Publication927 page 17 at https://www.irs.gov/pub/irs-pdf/p527.pdf#en_US_2020_publink1000219175 Read the “Example” in the third column.
Days of Personal Use - This number will be a big fat ZERO. Read the screen. It's asking for the number of days *YOU* lived in the property AFTER you converted it to a rental. I seriously doubt (though it is possible) that you lived in the house (or space, if renting a part of your home) as your primary residence, 2nd home, or any other personal use reasons after you converted it to a rental.
Business Use Percentage. 100%. I'll put that in words so there's no doubt I didn't make a typo here. One Hundred Percent. After you converted this property or space to rental use, it was one hundred percent business use. What you used it for prior to the date of conversion doesn't count.