What are the most expensive MEP aspects of converting a garage to an ADU?

The two costliest MEP variables in a garage conversion are almost always the plumbing — because a complete drain-waste-vent system has to be cut into the concrete slab — and the electrical service question, specifically whether the main house panel can carry the ADU or needs an upgrade. Both are construction and utility costs, not plan costs, but a well-designed plan set anticipates them so they don't appear mid-build.

The big-ticket items, ranked

Why plumbing is the most unpredictable line item

Garages sit on a slab-on-grade with no below-slab drainage. Adding a kitchen sink, a full bathroom, and a water heater means cutting and trenching the slab to install drain lines, P-traps, and vent stub-ups below grade, then patching the concrete back. Several site conditions drive the cost:

• Elevation relative to the sewer lateral — if the garage floor is at or above the lateral, drains must run deeper to hold the required 1/4-inch-per-foot slope, or a sewage ejector pump is added.
• Distance to the lateral — a garage at the back of a deep lot can require significant trenching.
• Post-tension slab — cutting one can sever tensioning cables and requires structural assessment before any work begins.
• Lateral condition — a pre-start sewer camera inspection is strongly recommended; discovering a failed lateral after the slab is cut is an expensive surprise.

The electrical service is the schedule risk as much as the cost

The biggest electrical cost variable is whether the existing service can carry the new ADU. A 200-amp panel with open slots may need no upgrade; a 100-amp service, or a 200-amp panel already near full load, often points to a service upgrade. A panel upgrade requires the utility to coordinate a power shutoff at the meter, and that utility timeline runs separately from — and often longer than — the permit itself. Identifying it early in the plan stage keeps it from stalling the project near completion.

Watch also for sewer-capacity fees charged by some districts, water-service sizing, and EV-ready conduit (cheap now, expensive to retrofit after walls close). Requirements and fees vary by jurisdiction and water district — confirm with your local building department and water provider.

None of these construction costs are part of the plan price. The MEP plans themselves are flat-priced and scaled by ADU size — single discipline from $995, two from $1,195, Full MEP from $1,495 — with no sales tax. A coordinated plan set is what keeps these big-ticket items from becoming surprises; see the cost of ADU MEP plans or start your order.