You have to allocate the special assessment between personal use and rental use. Then you also have to know what it is for.
A special assessment for repairs or maintenance is not personally deductible but is a deductible rental expense.
A special assessment that is for improvements is not personally deductible, but it increases your cost basis and ,ay reduce your gains when you sell. On the rental part, it is treated as an improvement, which is added to the cost basis and then depreciated year over year.