Indoor architecture like interlinked rooms is usually tricky in Blender, without help from heavyweight plugins. You want not to be destructively carving out your building shell, extruding geometry, or adding overlapping walls. You need a way to create internal and external shapes without committing to complicated meshes that are difficult to edit afterwards.

Here, without plugins, you’ll combine room shapes, windows, and doors, into a continuous and smooth internal shape that transitions effortlessly to the outside space. After, you'll still be able to freely add or remove rooms and features without fixing up the model afterwards.

If you want to decorate rooms separately with different wall textures, then you’ll want to work with a variation of this method to use individual internal skins, and use the windows and doors in all rooms.

In this workflow, you create collections of objects that you combine with modifiers. It’s possible for you to do this with fewer collections if you’re comfortable doing so.

Prepare the scene structure

You’ll need two objects:

• Outer skin. This is the outer shape of your building’s walls. Do not cut out windows, as you’ll do this later. Also omit added modelling, like the tiled roof, felt, and stonework.
• Inner skin. This is the outer extent of the internal decoration. You’ll usually want this a consistent distance inside the outer skin.

At the interface of the Outer skin and the Inner skin, you’ll later add joinery or frames to hold windows or doors.

First create some collections:

• Building, which will contain everything. Then inside that:
  • Rooms
  • Windows
  • Internal doors

Next, in Building, create a couple of cubes:

• Root inner skin
• Root outer skin

You’ll be adding modifiers to these, but not using their shapes directly, so move them out of the way and make them small, say under the ground.

Add modifiers to the Root inner skin object:

1. Add a Boolean modifier
   • Name: Add inner skin
   • Type: Union
   • Operand Type: Object
   • Object: Inner Skin
   • Solver: Fast
2. Add a Boolean modifier
   • Name: Cut rooms
   • Type: Difference
   • Operand Type: Collection
   • Collection: Rooms
   • Solver: Fast
3. Add a Boolean modifier
   • Name: Cut internal doors
   • Type: Difference
   • Operand Type: Collection
   • Collection: Internal doors
   • Solver: Fast
4. Add a Boolean modifier
   • Name: Cut windows
   • Type: Difference
   • Operand Type: Collection
   • Collection: Windows
   • Solver: Fast

Optionally, if you can make them work with your geometry, add modifiers to give your rooms slightly rounded edges:

1. Solidify
2. Bevel

Add modifiers to the Root outer skin object:

1. Add a Boolean modifier
   • Name: Add Outer skin
   • Type: Union
   • Operand Type: Object
   • Object: Outer skin
   • Solver: Fast
2. Add a Boolean modifier
   • Name: Cut inner skin
   • Type: Difference
   • Operand Type: Collection
   • Collection: Inner skin
   • Solver: Fast
3. Add a Boolean modifier
   • Name: Cut windows
   • Type: Difference
   • Operand Type: Collection
   • Collection: Windows
   • Solver: Fast
4. Add a Boolean modifier
   • Name: Cut rooms
   • Type: Difference
   • Operand Type: Collection
   • Collection: Rooms
   • Solver: Fast

Add rooms and holes

Now you can add:

• Room shapes to the Rooms collection.
• External doors and windows to the Windows collection. Use blocks that cross the external and internal faces of the walls.
• Internal doors to the Internal doors collection

You can add more to these collections without committing to destructive mesh detail that would be difficult to edit later.

Ensure that the floor level of the internal doors are not the same as the floor level of your rooms, because Blender’s Boolean modifiers cannot reliably compute overlapping planes.

Finally

To complete your building, add extra geometry to the basic skin you’ve just made: add joinery, frames, windows, roofs, internal decorations, and furniture. Remember to add some good lighting and render with Cycles and Filmic color management for a more realistic result.