Shade is the killer. Stand on your terrace at 10 am, noon and 3 pm - water tanks, parapets, taller neighbours and even a single tree branch can cut a panel row's output badly. You want a patch that stays sun-lit through the middle of the day, ideally facing south.
Space: budget roughly 80-100 sq ft per kW for standard panels. A 3 kW home system wants ~250-300 sq ft of clear, strong, accessible roof. Sheet roofs and tin sheds can work with the right structure but need extra care.
Ownership and wiring matter too: you'll need consent if the roof is shared, a sanctioned load that accommodates the system, and a meter location the DISCOM can reach. If all three check out, you're ready for site survey - which any good dealer does free.