The base cost of a starship without any components installed is determined by the number of 5 ft. x 5 ft. squares that it occupies. Expanding a starship's chassis cost 20 Credits for each additional 5-foot square space added, not including labor costs priced by a mechanic. This cost also applies to the construction of entirely new starships.
For example, a 40 ft. x 20 ft. starship chassis occupies thirty-two 5 ft. squares and costs 640 Credits to build. To lengthen this starship chassis to 50 ft. x 20 ft., you must pay 40 gold: 20 Credits for each additional 5 ft. square space.
Rooms and Walls. Once your chassis is created, you can apply any configuration of walls to divide the starship's interior into smaller rooms. Walls serve as excellent barriers against spreading fire or hull breaches.
Doors. With walls in place, add doors to the walls. There is no limit to how many doors you add in general, but they can impose challenges to traversing the starship if you add too many. At GM's discretion, you may need to spend 5 additional feet of movement, an action, or a bonus action to cross through a door.
Entrance. A starship's entrance can be located anywhere on the edge of the ship, including the ceiling, but most ships typically locate them at an airlock.
Living Spaces. You can eat and rest in any space of the starship using bedrolls, but you may choose to add bunks, tables, or other furniture to make it more comfortable.