What types of ADU projects need MEP plans?

Nearly every ADU project requires mechanical, electrical, and plumbing information as part of permit review. What varies is the scope — whether you are building systems from scratch or extending existing ones — and which disciplines a particular city reviews. Below is how the most common project types map to MEP scope. As always, requirements vary by jurisdiction, so confirm with your local building department.

By project type

• Detached ADUs — A new standalone structure has no existing systems, so it requires complete mechanical, electrical, and plumbing design from scratch, including a new service or subpanel and a new sewer and water connection.
• Attached ADUs — Additions connected to the main house need full MEP plans. Some systems may tie into existing house infrastructure if there is spare capacity, which the load calculation must demonstrate.
• Garage conversions — Existing garage wiring, ventilation, and (usually nonexistent) plumbing are almost never adequate for residential occupancy. These nearly always require full MEP redesign — see Garage Conversion MEP.
• JADUs (Junior ADUs) — Up to 500 SF created within the existing home. MEP requirements vary by city, but at minimum electrical and mechanical plans are typically required, even when the JADU shares plumbing with the main house.
• Additions — Any new conditioned square footage needs MEP drawings for the new systems serving that area.

A quick way to judge your scope

1. Is it a brand-new structure? Expect full M, E, and P from scratch.
2. Is it an existing space being converted? Expect full redesign, because the existing systems were built for a different use.
3. Is it carved out of the main house? Expect at least electrical and mechanical, with plumbing depending on whether new fixtures are added.

Manufactured and prefab ADUs

Factory-built and panelized ADUs are increasingly popular, and they still need site-specific MEP coordination. Even when the unit arrives with internal rough-in, the connections to the property — the electrical feed, the water supply, the sewer tie-in, and the gas line if any — must be designed and permitted for your lot. The factory drawings rarely cover those site utilities.

When a single discipline is enough

Occasionally a project only triggers one or two disciplines — for example, a JADU that reuses the existing kitchen and bathroom may need electrical and mechanical but little new plumbing. We offer single-discipline plans starting at $995 and two-discipline sets from $1,195 for exactly these cases, while the Full MEP Package starts at $1,495 when all three are needed.

If you are unsure which disciplines your city will require, the California ADU Requirements page is a good starting point — or send us your project details on the order page and we will scope it for you.