Does a garage conversion require a fire separation between the ADU and the main house?

Often, yes — but it depends on the configuration. When a garage shares a wall, floor, or ceiling with the main house (an attached garage or one beneath living space), building codes generally require a fire-resistive separation between the new dwelling and the existing house, and any MEP penetration through that assembly must be fire-stopped. The exact rating and where it applies vary by configuration under the CRC/CBC, so this is a point to confirm with your local building department.

When fire separation typically comes into play

• Attached garage conversions — a garage sharing a common wall with the house generally needs a rated separation between the two dwelling units, similar in principle to the separation already required between a garage and a residence.
• Garages under living space — a conversion beneath an occupied floor commonly requires a rated ceiling/floor assembly.
• Detached garage conversions — a freestanding garage usually has no shared assembly, so a unit-to-unit fire separation often doesn't apply, though proximity-to-property-line and opening rules can still trigger rated exterior walls.

Why this matters to the MEP plans

Even where the architectural drawings define the rated assembly, the MEP scope is directly affected because every pipe, duct, refrigerant line, and electrical penetration through that wall or ceiling must maintain the assembly's integrity:

• Mechanical — refrigerant lines or ducts that penetrate a fire-rated wall between the ADU and the main house must be fire-stopped, and that detail is shown on the mechanical plans.
• Electrical — conduit and cable runs crossing the assembly require approved firestop systems, noted on the electrical sheets.
• Plumbing — supply, drain, and vent lines passing through the rated assembly need rated penetration details.

Missing fire-separation detailing — penetrations through a required fire-rated wall not shown as fire-stopped — is a common plan-check correction on conversions. Showing the firestop details up front avoids a correction round.

A practical checklist

• Confirm the conversion's configuration (attached, under-house, or detached) and ask the building department which rated assembly applies.
• Identify every MEP penetration through that assembly during design, not after.
• Show approved firestop details for each penetration on the relevant trade sheet.
• Coordinate the rated assembly between the architectural drawings and the MEP plans so they don't conflict.

Fire-separation specifics depend on your configuration and local code, so confirm the rating and locations with your local building department. Our Full MEP Package coordinates the mechanical, electrical, and plumbing penetration details around your architect's rated assembly; start your order or see how it works.