A vertical or horizontal addition to any building, regardless of historic status, must conform to the Residential Design Guidelines.
Additions to known historic resources must also meet the Secretary of the Interior's Standards and Guidelines for the Treatment of Historic Properties and any applicable requirements outlined in Article 10 of the Planning Code. Generally, if an addition does not alter, change, obscure, damage, or destroy any character-defining features of a historic resource or building located within a historic district then it will be deemed in conformance. The Planning Department strongly urges project applicants to engage with a Historic Preservation Technical Specialist early in the design process, in order to identify potential limitations.
The CityStructure Report tells you in which historical category falls a property and if you could build additions to the existing building.