You may find lots of useful in formation in this IRS publication:
https://www.irs.gov/publications/p527
If it had been an qualified maintenance/repair for the purpose of readying it for rental, it would be allowable (at least that in the year you had it available for rental). The problem is that this construction sounds more like a capital improvement. You don't get to just take that off the rental income, but it will increase your basis when you sell the property and will affect depreciation deduction that you should be taking.
I am going to explain that a little differently because the OP may not understand that.
The money you spent converting the property into rentals is something that you would depreciate rather than expense. Depreciation means that you take a portion of the expense on a yearly basis until its all used up. Since this is your first year renting out the space you would add it to your basis of the property and that would make it part of the annual depreciation expense that you take for the property.
Here is an example. You paid 150,000 for the property and made 50,000 in capital improvements making it suitable for apartments. That makes your basis 150 +50 =200,000.
The depreciation life of a rental property is 27.5 years, so you would divide 200,000 by 27.5 = 7273.00 and that is the amount that you would expense yearly. This is a simplistic explanation. You will want to a tax professional to assist you with your tax return.
