CT Book 2 Starships p. 21 "Finally a leeway of plus or minus 10% to 20% should be allowed. If the final deck plans come within 20% of the tonnage of the ship specifications, then they should be considered acceptable."
IIRC the hull is 2,000 tons deck plans adding to 2,400 tons is acceptable. Current deck plans 3,000 tons contact Grandfather and barrow a device that negates 600 to 1,000 tons.
Thats a good thought if those margins are still allowed! Of course Mongpose doesn't mention the 2 squares a ton (that I can recall) and T5's *table* of tons to squares looks a bit strict for that fudge! but I might well take advantage of it anyway when it comes to having to rip out the luxury fittings!
The 20% leeway allows for things like fins, hull bulges and streamlining, which use volume but no deck plan squares on the one hand and on the other hand reduced headroom areas and especially reduced inter deck spaces which mean that each deck square represents a lower volume. It also hides the fact that there is no overhead for cargo in the rules: 100dT of cargo requires 200 deck plan squares.
Personally I'd just make the ship bigger and include extra "staterooms" to cover the swimming pool etc. instead of assuming that it comes from the common area of the existing stateroom volume.
Alternatively, you could make it a maintenance nightmare! Minimum inter deck spaces, just enough to hold the grav plates and the life support, lots of automated repair but it needs a week at an A or B star port after every jump and quarterly, not annual, maintenance; plus, if there's a problem, it's not Scotty in the Jeffries tubes, it's draining the swimming pool to get at the access panel for the dehumidifier or having to move the dining table during lunch because the leg is on top of the overheating electrical junction.
Phil Kitching