How much does MEP planning and engineering cost for a garage conversion vs. a new detached ADU?

For MEP plans, a garage conversion costs essentially the same as a new detached ADU — the same flat pricing, scaled by square footage — because the engineering scope is comparable. The systems are designed from scratch either way, so an existing garage structure doesn't reduce the design work. What differs between the two project types is the construction cost, not the plan cost.

Our flat plan pricing (same for both project types)

Pricing scales by ADU size, there's no sales tax, and orders include two revisions plus city corrections. Turnaround is 5 business days standard, with Priority (3 days, +$299) and Express (1-2 days, +$599) options.

Why the plan scope is the same

It's tempting to think an existing garage shortcuts the engineering. It doesn't:

• Electrical — both projects need a load-calculated subpanel, residential circuits, GFCI/AFCI, EV-ready conduit, and detectors. The garage's few existing circuits don't count toward that.
• Plumbing — both need a full kitchen-and-bath DWV and supply design. The garage has no plumbing to reuse; in fact, its slab adds the under-slab cut.
• Mechanical — both need a mini-split and ASHRAE 62.2 ventilation designed from scratch.

If anything, a conversion adds a few design considerations a new build doesn't have — the under-slab DWV routing, the ventilation that replaces the lost garage-door airflow, and any fire-separation penetrations to the main house — which is why the plan pricing is the same rather than lower.

Where the real cost difference lives

A new detached ADU carries foundation, framing, and full-shell construction costs a conversion avoids — that's where a conversion genuinely saves money. But the MEP construction for a conversion has its own variables: slab cutting ($2,000-$5,000), a possible service upgrade ($3,500-$7,000), and long supply/drain runs to a back-of-lot garage. These are construction and utility costs, not plan costs.

So the honest comparison is: the same MEP plan price, lower overall construction cost for a conversion, but conversion-specific construction variables to budget for. Requirements and local fees vary by jurisdiction — confirm with your building department. See full pricing, the cost of ADU MEP plans, or start your order.