What does MEP stand for, and what does each letter cover?

MEP stands for Mechanical, Electrical, and Plumbing — the three engineering disciplines that govern every active system inside a building. While architectural plans describe the shape and layout of your ADU, MEP plans describe the systems that actually make it livable: how it stays warm and cool, how power and light reach every room, and how water arrives and waste leaves. On a permit set these are the sheets a plan reviewer turns to in order to confirm your new dwelling is safe, healthy, and energy-compliant.

Mechanical (M) — heating, cooling, and ventilation

The mechanical discipline covers comfort and indoor air quality. For a California ADU this almost always means a heating and cooling system — most often a ductless or ducted mini-split heat pump — plus the ventilation required by ASHRAE 62.2 and Title 24. Mechanical scope includes:

• HVAC equipment selection, location, and capacity, sized from a Manual J heat-load calculation
• Whole-dwelling mechanical ventilation — a continuous exhaust or supply fan sized to the unit
• Bathroom exhaust fans, kitchen exhaust, and condensate drainage
• Duct routing, register locations, and equipment schedules where a ducted system is used

Electrical (E) — power, lighting, and circuits

The electrical discipline covers everything that draws current. Because an ADU is a separate dwelling, it typically needs its own load calculation, panel or subpanel, and a complete circuit layout drawn to the California Electrical Code (CEC):

• Outlet, switch, and lighting plans with GFCI and AFCI protection noted where required
• A panel schedule listing every branch circuit, plus a service or feeder load calculation
• Dedicated circuits for HVAC, range, dryer, water heater, and — where applicable — an EV charger
• Smoke and carbon-monoxide alarm locations and any solar-ready conduit

Plumbing (P) — water, sewer, and gas

The plumbing discipline covers all wet systems under the California Plumbing Code (CPC):

• Water-supply lines, fixture locations, and a fixture schedule with flow rates
• The drain-waste-vent (DWV) system and the connection to sewer or septic
• Water-heater specification and any gas piping
• CALGreen low-flow fixture compliance

How the three fit together

Although M, E, and P are distinct trades, they are designed as one coordinated package. The water heater appears on the plumbing sheets but draws a circuit shown on the electrical sheets; the heat pump appears on the mechanical sheets but needs both a dedicated circuit and a condensate route. A good MEP set resolves these overlaps before submittal so the disciplines never contradict one another.

Think of MEP as the nervous, circulatory, and respiratory systems of your ADU. Architectural plans draw the body; MEP plans make it function and keep it code-compliant.

Each discipline is covered in depth on its own page — see Mechanical Plans, Electrical Plans, and Plumbing Plans — or review the Full MEP Package when you need all three coordinated together. Exact requirements vary by jurisdiction, so confirm specifics with your local building department.