Also, remember how huge a 20x20 room is. There are options for smaller rooms in there that would take up less space. Also, remember there are heavy price modifiers depending on location.
How big is the city?
How far outside of it are they?
What is the climate/terrain of the area?
Do the PC's have access to any of the creation spells? (Airwalk, Fly, Levitate, Fabricate, Move Earth, Stone Shape, Telekenesis, Wall of Stone, Wood Shape)
So the cost for Bedrooms (x4)(2ss) is 1400gp.
A 1ss kitchen is capable of preparing meals for up to 15 people. Also a 20x20 kitchen is gynormous for a smallish house. I'd reduce it to .5 ss, eliminate the cook prerequisite, and drop the price to 1,000.
And a Basic Common Area costs 300gp.
Basic Bath is 400gp.
Basic Storage is 250.
Total Room Cost: 3350.
There's no cost adjustment for height if they only want the house to be 2 stories. Spaces in the 1st level of the basement, also do not incur a height adjustment cost.
Walls: You can mix and match your wall types. The ground floor with wood walls is free, and in the basement, hewn stone walls are free. Save money on the second floor and transition to packed earth (500gp total) or save more money and don't have a 2nd floor. Your kitchen can go into the basement with proper ventilation and you can easily put all of your ss on the ground floor should you want to. If you do pay for wood walls, and the terrain is forest, don't forget that you can take 10% off of the price.
Or look at it this way. A Basic Residential structure for 30 people costs 12000. That's approx 400gp per person. Multiply by 4, and you have 1600.
The total cost for the house below is 3500 before any adjustments. The second floor has packed earth walls; the first floor has wood walls, and the basement has hewn stone walls. It includes 4 bedrooms, a bath, common area, kitchen, and storage.
If the climate is warm, located on a plains area, the price is reduced. It's also altered by how far the house is outside of the city.
*The map below was made using Dundjinni software.