Multiple Buildings on One Lot

Last updated April 10, 2025
By Ian Story

This article discusses the code implications of having multiple standalone buildings on one lot. Specifically, we will investigate fire separation requirements and code-compliant options to reduce these requirements.

Fire Separation

The basic fire separation requirements for a building are based on the fire separation distance for each exterior wall. Here is the basic definition from the IBC:

The first key point to note is that, when using measurement #3, you as the designer get to pick where that imaginary line is located. Say you have an existing house and a new ADU located 9 feet apart. If you put the fire separation line directly in the middle, both walls will have a fire separation distance of 4.5 feet and will need to be fire rated. If you instead put the fire separation line 5 feet away from the existing house, only the ADU wall would need to be fire rated and you would not need to make any upgrades to the existing house. (note: this example assumes both structures are R-3 occupancies and only require fire rating for a fire separation distance of less than 5 feet).

However, there is an even better trick available. IBC Section 503.1.2 gives you the option to classify separate buildings on the same lot as portions of one building.

IBC 503.1.2 BUILDINGS ON SAME LOT: Two or more buildings on the same lot shall be regulated as separate buildings or shall be considered as portions of one building where the building height, number of stories of each building and the aggregate building area of the buildings are within the limitations specified in Sections 504 and 506. The provisions of this code applicable to the aggregate building shall be applicable to each building.

For applying Section 503.1.2, each building checks building height and number of stories individually. Only the total building area is considered in the aggregate. For example, we are designing a 4-story single-family residence next to an existing 2 story triplex on the same lot. The new structure is sprinklered but the existing one is not. For type VB construction, buildings are limited to 3 stories without sprinklers. Considered as a single building, this would not comply (the highest story is 4, but the whole building is not sprinklered). But Section 503.1.2 allows us to check each building’s height and number of stories separately, while still considering them portions of one structure for applying fire separation requirements (as long as the combined building area doesn’t exceed the limits of Table 506.2).

References & Interpretations:

• MeyerFire Forum: Does IBC Ch. 9 Apply to 2 Buildings on Same Lot?

If the two buildings can be considered portions of one building under Section 503.1.2, then you no longer need to worry about fire separation between the two structures. See IBC Section 705.3:

References & Interpretations:

• The Building Code Forum: 705.3 Buildings on the same lot exception question