RESIDENTIAL CONSTRUCTION
Government Code 65995(b)(1)
a. Developer fees are calculated on all new residential construction.  All covered assessable space within the perimeter of a structure. 
b. Any residential additions to existing structures resulting in an increase in assessable space exceeding 500 sq. ft. are also subject to the collection of developer fees.  This is an accumulative number.  In other words, the 500 square feet minimum is applied to the building throughout the course of the structure’s life.   

Exclusions from assessable space include parking structures, carports, walkways, garages, overhangs or patios, detached accessory structures or similarly enclosed areas, or additions of 499 sq. ft. or less.

COMMERCIAL/INDUSTRIAL CONSTRUCTION
Government Code 65995(b)(2)
For purposes of determining all chargeable covered and enclosed space, any exterior space which may be covered and/or enclosed at any point during the year shall be determined to be an extension of such structure intended to expand the use of such structure, whether or not said exterior extension is within the perimeter of a commercial or industrial structure. This space may include exterior dining areas, dancing areas, and play areas which have the ability to be covered and/or enclosed in a manner to extend the use of the facilities and require an appropriate building permit from the building department of the city or county.  Any additions to existing structures resulting in an increase in assessable space are also subject to the collection of developer fees.

This includes: hotels, inns, motels, senior citizen housing (restricted to 55 years old and over, Civil Code, § 51.3), residential care facilities for the elderly (Health & Safety Code, § 1569.2(k)), and multilevel facility for the elderly (Gov. Code, § 14432(d)(9)).

Exclusions: This will not include any storage areas incidental to the principal use of the construction, garage, parking structure, unenclosed walkway, or utility or disposal area.

EXCLUSIONS
Pursuant to Government Code 65995 fees cannot be levied on a facility that is used exclusively for religious worship, owned and occupied by state, federal, or local government, or is used exclusively as a private full-time day school as described in Education Code Section 48222.  If you are replacing a structure destroyed by a disaster, you may also be exempt from fees.