Do you live in an area that requires permits? There are a ton of unpermitted garage conversions in my area that are not up to code, some of which have become serious headaches for the owner if code compliance comes knocking or when trying to sell.
Is your HVAC rated for the additional sq ft? Are there any ventilation concerns? (eg gas furnace or water heater)
I’ll definitely be getting permits and inspections. Went through that process when converting the bonus room and it wasn’t as painful as I was expecting.
The HVAC question is something I’ll definitely have to look into. I added ~250sqft when I finished my bonus room, and although the system has operated fine with the additional load, adding another 400sqft might put us outside it’s abilities.
Our appliances are all electric so there are no combustion related vents.
Edit: However, I am seriously contemplating installing a zero clearance wood burning fireplace in the room. We have one in another room and I love it. That is a topic that will come with it’s own set of wrinkles (chimney case framing, blower vents, outside air intake, etc). I’ve yet to make a decision on that but will delve deeper into it if/when I do.