@Christopher Chu wrote:
Then, at the prompt for Improvement (e.g., the building value), I mistakenly thought TurboTax asked for the Improvement that I'd made to build 2 small rooms in the basement, I enter 10K instead of the (actual purchased value - 24K).
Depending on your perspective, that could be viewed as you did NOT depreciate the main building at all. You only deprecated the separate 'improvement' (the two rooms). It is an arguable point, but I think I would try it
IF that is the case, then you can use Form 3115 to 'catch up' on all of the depreciation.
I recommend to file an extension, and contact a tax professional in May about filing this year's tax return. When looking for a tax professional, ask if they know how to use Form 3115 to 'catch up' on missed depreciation.
