When to develop a rental property from Duplex to Triplex

[Thomas Corey]

We have a duplex and one of the units (2bd/1ba) is in sorry condition and may be nearing the end of its useful life - as in, its foundation would take 50-60k to fix and the wood siding needs to be replaced at not an insignificant cost. My question is: how do I know when it's time to tear it down and build a new rental? We think we have enough room to make the current rental unit into two separate rentals making our duplex property into a triplex.