While the mansion is supposed to be the same building between the games, the layout is totally different.
The following rooms were present in the original, but not Day of the Tentacle:
- Underground, below the house
- 1st Floor, Living Room
- 1st Floor, Library
- 1st Floor, Dining Room
- 1st Floor, Pantry
- 1st Floor, Pool
- 1st Floor, Garage
- 2nd Floor, Landing
- 2nd Floor, Music Room
- 2nd Floor, Art Room
- 2nd Floor, Arcade
- 3rd Floor, Dark Room (where you develop photographs)
- 4th Floor, Dr. Fred's Bedroom
- 4th Floor, Dead Cousin Ted's Room
- 4th Floor, Dead Cousin Ted's Bathroom
- 4th Floor, Study
- 5th Floor, Observatory
- Basement, Dungeon
- Basement, Outer Lab
The following rooms are new to DotT:
- The Convention Center
- The Laundry Room
- Guest Room #1
- Guest Room #2
The following rooms moved around:
- Dr. Fred's Office: 2nd floor in original, 1st floor in DotT
- Green Tentacle's room: 5th floor in original, 2nd floor in DotT
- Edna's Room: 4th floor in original, 3rd floor in DotT
- Ed's Room: 4th floor in original, 3rd floor in DotT
- The Kitchen is now through the convention center.
You may have also noticed that DotT's house has one less floor than Maniac Mansion's house. This is because the Dark Room was essentially its own floor, an oddity in the house's design.
All in all, Day of the Tentacle has less rooms in the Mansion, but is still the game with more rooms due to having to explore it in three time periods.
There's no in-game explanation as to why the Mansion is different between the games. You can't explain it away by saying they did construction on it, because the Mansion in the colonial times looks much like it does in the present.