Unless local requirements dictate otherwise, consider providing a variety of unit types - studios, 1, 2, 3 and four bedrooms. The proportion of each type should take into account the population being served and the prevalent mix of units in the area surrounding the project. In multi-story buildings, try to locate larger family units on the ground floor to allow easy access and surveillance of children.

This Sacramento townhouse development provides a wide mix of unit types. Pictured here are the two-bedroom model on the left in both illustrations and the four-bedroom on the right. Note how the design accommodates two distinct unit types and sizes directly next to each other, rather than segregating all the two bedrooms in one area, all the fours in another, etc. 
(Southside Park Co-housing)

The wide mix of unit types and configurations in this Tacoma, Washington development is indicated by the different letters and numbers used to identify them on the plan. 
(Matsusaka Townhomes)

This plan shows how three distinct apartment types - studios, two bedrooms and three bedrooms - can be accommodated on the same floor. 
(555 Ellis Street)

This site plan indicates the wide range of unit types (models A-J) that have been included in this moderate size (21 units) single-family development in St. Paul, Minnesota. 
(Lyton Park Place)