More likely, you need a slab or crawlspace, but you need to check your local building codes. There may also be restrictions on what percentage of your lot can be built on.

we once bought a house that had a one-floor-up deck to sunroom conversion. while their work didnt pass our home inspection (they didnt do the conversion to code, and never got a permit) they were able to hire a carpenter to beef up the underframing to meet code.

so its possible to do it. you might have an issue if your deck is more at grade level and you cannot get under it. find out what the code is from your town's building department and move forward from there. unless it's going to be 3 season only you'll want to make sure you have good insulation in the floor as well as the walls.