To build a room, select its type from the Build Mode menu. Make sure the building icon below is highlighted and then build the room wherever you want on the grid on the ground. To do so you have to press the left mouse button (or hold it and move the mouse, if you want the room to be bigger than one tile). Holding the Shift key allows you to build bigger rooms faster.
To build a second and higher floors, first press an up or down arrow on the panel on the left side of the screen, which changes the level of the floor you are viewing. The new grid shows where you can build the rooms, based on the size and shape of the floor below.
To expand an existing room, you have to choose the type of the room you wish to build and move your mouse to the tile on the grid next to the wall which you wish to expand. Hold the left mouse button and the room will be expanding in the direction of your choosing.
You can also click the expand icon next to the building icon. You can then build outside the room you wish to expand, and the two areas will automatically connect.
To remove a room, press the 'Remove' button next to the expanding room icon. While the button is highlighted, holding the left button on a chosen tile will remove the room occupying it (the mechanic is exactly opposite to basic building). You have to make sure that there is no room on the floor above the room you are deleting and no stairs leading to the room.
DOORS, WINDOWS, FLOORS, WALLS
The first two menus to the right of Rooms allow you to build and choose the style of doors and windows. Simpl
y select one and place it on the wall of your choosing. The next menu is Walls. You can choose there the style of a wall you want - just choose it and press on the wall you wish to change. Similarly to that, in the next menu, Floors, you can choose the style of the floor. If you hold the Shift button when changing the style of a wall or a floor, it will automatically change all of the walls/floor in the given room.
The last option lets you choose the style of the roof of your inn.