San Diego ADU - Setbacks per Bulletin 400

[Peter Lee]

Looking at building an ADU and in a recent information bulletin the city of San Diego noted -

2. A companion unit may encroach within the interior side and rear yards as follows:

a. A structure, up to a maximum encroachment of 30' in length.

How exactly do I interpret this - typical setbacks are 5' on the sides and 15' in the back. If I "encroached" on the sides and back 100% would be at 25' of encroachment which is how read it but that makes no sense. 

Does anyone know exactly how to interpret this.