Little Big Workshop: building new rooms
You'll probably want to start expanding after an in-game week or two and doing so is pretty easy and cheap, but the way to do so with the construction tools (walls, doors, and bulldozer) isn't perfectly obvious.
In this short tutorial, I will show an example where I expand the initial room to make it bigger. Doing so requires using all three construction tools.
So the only rule is that the new room that you want to build must connect to a door. If there is no door, you won't be able to pay and order the room's construction.
Here is my starting workshop and the area where I want to expand the small room with a door to the loading area:
You'll notice no door connects to that red square and that the walls are full of workstations. So the very first step is to move things around and to build a connecting door.
Building the room
Once a door connects, the game allows me to create the new room:
A room of that size costs around 2000 gold.
A minor detail that is good to know is that the construction workers bring their truck to the loading area and will occupy it until the new room is completed. It could impact your production if the new room is super huge.
Once construction is completed, all I have to do if I want to merge the new room with my initial one is to use the bulldozer tool on the internal wall:
Finally, I'll right-click the connecting door and destroy it.
I now have a large room which I intend to use as a central room for assembly and storage.