Fast level rooms with arched hallway

Make the 2 room foundations as splines in top view.

Make the spline path for the hall.

Select both rooms and extrude up.

Convert rooms to editable ploy.

Take the bigger room and raise the ceiling.

Make a layer for each item.

Hide big room, lock small room.

In front view make a 10 sided ngon. This will be used to make the doorway.

Convert to editable spline.

Select all vertices and right click to make all vertices corners.

Remove the bottom half of the ngon.

Turn on connect copy.

Shift drag down the bottom spline

 

Delete the original spline

Arc rotate to position door over spline path for hall.

Extrude spline to 0, and convert to editable poly.

Extrude door on spline.

Bottom view, unfreeze small room, insert 2 vertices to match up to the hallway.

Attach the hallway and small room as one object.

Delete the small room wall facing the hallway.

Target weld hallway vertices to small room inserted vertices.

Cap the hole on the small room.

Under geometry, make the new wall planar:

Same procedure for large room.

Select the appropriate polygons and assign IDS for

Floor = 1

Walls = 2

Ceiling = 3

Make a test multi-sub with 3 bright colors and apply.

Select all elements and flip normals [under edit element rolloout]

Next step is to make the final material and work with the UVWs.