Tax Question Regarding Unallowed Losses

[Gabe S.]

Quick tax strategy question.

I have accumulated over $50K in "unallowed losses" on my tax returns over the past few years.  What happens to those losses when I sell the property? Does it get added back to the basis? Do I just lose it?    I plan to sell the property in a year or two and am starting to think about whether 1031 or not. What happens with the UL could (I think) impact that decision.  Any insights?  Thanks in advance.