What common garage conversion MEP mistakes delay permits?

Most garage-conversion permit delays come from a handful of avoidable MEP and energy-code oversights — incomplete electrical documentation, plumbing that ignores the slab, missing ventilation, and the wrong Title 24 compliance path. Getting these right up front is the single biggest factor in passing the completeness review and avoiding correction rounds that each add weeks.

The most common plan-check triggers

1. No slab-cutting plan for plumbing — plans that don't account for the fact that all DWV must go below the slab, leading to conflicts discovered during rough plumbing. The plumbing routing must be coordinated with the electrical and floor plan from the start.
2. Garage heater not shown as removed — the existing non-residential heater must be explicitly noted as removed, with the replacement system clearly specified.
3. Panel capacity not analyzed — assuming the existing sub-feed to the garage (often 60 amps or less) is adequate, with no load calculation justifying the subpanel size or confirming the main service can carry the ADU.
4. Ventilation not added — closing the garage door opening without designing ASHRAE 62.2 whole-building ventilation to replace the lost air exchange.
5. Existing outlet spacing accepted as-is — garage outlets don't meet residential spacing; every wall must be evaluated for code-compliant outlet coverage.
6. Wrong Title 24 compliance path — applying new-construction rules when alteration rules apply, or submitting an unregistered CF1R or the wrong climate zone.

Documentation and coordination mistakes

• Incomplete submittal — a complete package (registered CF1R, full MEP plans, coordinated architectural/structural drawings, required forms) passes the completeness review and enters plan check immediately; a missing piece stalls it before review.
• Uncoordinated disciplines — outlet, fixture, and equipment locations that conflict between the architectural, electrical, and plumbing sheets, or HVAC specs that don't match the energy report.
• Missing fire-separation detailing — penetrations through a required fire-rated wall to the main house not shown as fire-stopped.
• Skipping the sewer lateral check — discovering a failed or undersized lateral after the slab is cut.

The most important timeline factor is application completeness. A coordinated, complete submittal is what lets a conversion clear the 15-day completeness review and enter plan check without bouncing back. Each correction round can add roughly two to six weeks.

A pre-submittal self-check

• Does the plumbing layout assume below-slab routing and show the slab cut?
• Is the existing heater noted as removed, with the replacement specified?
• Is there a load calculation justifying the subpanel and confirming the main service?
• Is ASHRAE 62.2 ventilation designed to replace the lost door-opening airflow?
• Do outlet locations meet residential spacing on every wall, including the closed opening?
• Is the CF1R registered, on the alteration path, and matched to the right climate zone?
• Do the HVAC, water heater, and lighting specs match between the MEP plans and the CF1R?

Requirements vary by jurisdiction — confirm the exact submittal checklist with your local building department. Using a single team for mechanical, electrical, plumbing, and Title 24 eliminates most coordination corrections, and our orders include two revisions plus city corrections. See the complete ADU MEP guide, review pricing, or start your order.