You will have a load change because of the added lumber
this may mean you need to sister on joists under the room
it may also mean you need to change the center beam

you will probably leave the subfloor if it is plywood and
good condition but you may need to cut each new joist
you use to build the box.

unless you are modifying the home for handicap access
I would suggest living with it. or pic a different house


wrote in message
oups.com...

Im looking into buying a home whose 17' by 15' living room is approx 6
inches stepped down from the other floors. There is an unfinished
basement beneath this room and more than 10ft of ceiling above this
room.