BG, you can build a granny flat for about $200/sf. If you build a 550 sf guest house, that’s $110,000. Assuming a mortgage can be had a 5% (easy to get right now), that would give you a $590.50/month P&I mortgage payment. Add additional maintenance, tax, and insurance costs (~165/month), and you’re looking at somewhere around $755.50/month in additional expenses for the accessory dwelling unit (ADU). Around here, you can *easily* get $900/month for such a unit, so it makes sense for many people to do this. It just depends on whether or not one needs/wants to do it, and whether or not they have the money or equity in their existing home to use for the construction of the ADU.
