Is my outdoor deck floor strong enough to support a new room on my house?

Depends on how it is attached to the house? Is there a leger board running across the side, that everything else is connected to? Has it been lag bolted into the house, or just nailed? The leg posts should be replaced with concrete pillars or blocks to hold up all the wight you will be adding. Need to have an city inspector come out and take a look at it.

I agree with the tear, off start fresh. A good inspector will want to see footings that the piers are sitting on regardless. In theory though a deck in my state has a live load minimum off 50lbs per sq ft which is the same load that a living room is required to have. It may be possible to get an engineer to come out and look at what you have and sign his name to its integrity. That may be the only cheaper way to leave what you you already have and still be up to code.

Hi, Harri, greetings from the colonies. I live in New England.It gets a tad chilly here.. What are you going to use for wall studs??? If you use 2 x 4 construction use 4" insulation, like wise if you use 2x6 walls use 6" ins..Make sure,contractors are sloppy, that the insulation is tight to the top and to the bottom of each bay. Leave no holes or gaps. If you stuff 6" ins in a 4" bay it will compress the ins. If it is compressed it, in essence, becomes more like a solid and defeats the ins. value...You cannot make it to tight. The better the ins the better the comfort and more even the heat.. The attic should have about 10" of ins.. either blown in or in tightly fitting batts.Type in Owens Corning or whatever English companies there are and request info on their products..A little research now could save a bunch on your heating bill's..Hope you enjoy your new home..Bamboo floors, exotic hard material..Don't know a thing about it.....Buzz

I also think it's a redo. Your local building code office can tell you what's needed.

Just a guess: it's a complete redo.