The permit question comes first, because it determines everything else. If a second unit was never permitted, a lender will not count its income, and you are effectively financing a single-family home with an unauthorized extra space attached, which changes your numbers significantly.
Once legality is confirmed, the process mirrors a standard purchase: an accepted offer, a condition period for inspection and financing, and a possession date agreed between the parties. A home inspection, typically $400 to $600, should cover every unit in the building, not just the one currently vacant or shown.
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