Place a driveway and extension piece
Turn on the cheat boolprop constrainfloorelevation false
Use the small raise terrain tool and raise a 1x1 square of terrain by 8 clicks.
Drag from the raised 1x1 piece using the level terrain tool across the extension piece and 1 row of squares of the driveway (9 squares in length in total)
Should see below (with 6 squares of driveway showing above the pavement)
Using the small raise terrain tool, raise the first line of driveway by 6 clicks, then th next line by 4 clicks, then the next by 2 clicks, creating a slope.
Use the design tool and recolour the driveway and extension piece, this will make them show correctly as below.
Add walls around the extension piece
And finish your garage