Can I deduct adding a pool enclosure?

[KarenL2]

If it's for a rental property, yes, it's deductible.  It would be an improvement to the property.  If the cost is over $2,500 then enter it as an Asset.  Otherwise, you can claim an ordinary expense for it on Schedule E.

If it's for a personal residence, then you cannot deduct it.  However, you should keep the receipts as it can be added to the property basis value (as an improvement) when you sell to reduce potential capital gain.