Late adding building addition for CCA purposes

If someone lived in a condo for 2 years, moved out, and started renting the condo out, but didn’t add the condo as an addition as a capital asset for CCA purposes over the course of 5 years, but now wishes to start claiming CCA, how should they go about creating that addition? Would they have to go back and ammend their taxes from the year they started renting it out? Or could they just enter the addition for this year? Also, how would you go about determining the value of the addition? They purchased it for a certain price, but then lived in it for 2 years before renting it out.

My first question would be about the wisdom of claiming CCA on the condo.

I would suggest if they insist on claiming CCA, that they start now with the purchase value.

That someone should actually be going through all the math on a spreadsheet with his CPA before further decisions which will impact not only taxes payable, but limit or enable elections, which likely are already late.
In any event, IMHO insufficient information to comment. Would need to look at the whole file.