A Note: As far as I know, there is no cheat to make it so friezes can be placed on ground floors. For that, you should probably just do the traditional way to making modular stairs. Also, these stairs will not be equal length, so if you suffer from video game symmetry OCD, avoid this tutorial!
Friezes are a feature in Ambitions that allows you to place foundation height walls on top of normal height walls without having to go through the "constrainfloorelevation" cheat. They are acessed through a cheat called "placefriezes on/off." Friezes are created by going into the foundation menu, clicking the traditional brick foundation and dragging it across an upper level with a floor. Friezes cannot be placed next to walls unless you used the "constrainfloorelevation" cheat.
Games Needed:
Ambitions
Cheats Used:
placefriezes on/of
constrainfloorelevation false/true
Step 1
Create the shape of your house and plan where you want you rooms and stairs to be.
Step 2
Place floors on your second floor and partially on your third floor where you want the upper section of your modular stairs to reach. You need 2 squares of space free from the frieze to the upper floor for this to work.
Step 3
Open the cheat menu by holding ctrl+shift+C, type in "placefriezes on." Go to the foundation menu, choose the traditional foundation and place a 2x2 frieze on the second floor.
Step 4
The frieze counts as part of the third floor so go down one floor, hold ctrl and delete one square the way you would with foundations to give it an L shape.
Step 5
Choose your stairs and place it in the flat section of the L.
Step 6
Open the cheat menu again and type "constrainfloorelevation false" (remember to open the cheat menu again when you're done, hit the up key and replace false with true.) Choose your stairs again and place them by holding the stairs close to the third floor.
Other Shapes
Instead of doing U shaped modular stairs, you can make L shaped modular stairs by doing all the steps above but replacing the L shaped frieze with a basic 1x2 frieze.
You must do a 1x2 frieze. Although you can place modular stairs on a 1x1 frieze, it won't work in-game. Also, it looks stupid.
Here's an example of how I used this method in one of my houses.