Best way to make many 3D walls?

I'm making a 3rd person game and for this I need a level/environment. This level is essentially the layout of a building so its compromised of many walls separating rooms and hallways.

I know the layout of the level I want so I made a sprite of the layout and brought it into the game as a 3D box shaped as the floor. Now I want to build the walls on top of this floor but the only way I can see of doing this is creating a 3D box each time and carefully adjusting it to fit on top of my layout/with other walls.

With the map I'm trying to create that can be easily 100+ objects alone which looks like a ton of painstaking work. I've been trying to find a simpler solution but nothing for Unity, any suggestions would be appreciated.