What actually drives the cost of an ADU permit-plan package?

The property determines the starting difficulty

A new detached ADU on a relatively open, documented site is a different assignment from a garage conversion, hillside project, addition to an older building, or site with uncertain records and tight clearances. Existing conditions create documentation and coordination work before the proposed rooms are resolved.

The amount and quality of available information also matters. A current survey, reliable drawings of the existing building, clear photographs, and confirmed utility information reduce assumptions. Missing or conflicting records can require additional field work, research, or help from an engineer or other specialist.

Project type changes the drawing and analysis

Detached construction, attached additions, conversions, and junior ADUs involve different relationships to foundations, structure, fire separation, energy documentation, utilities, and existing occupancy. A customized design also requires a different level of exploration from a previously developed plan adapted within known limits.

The proposal should name the project type and intended rooms or use. If the homeowner has not chosen an ADU idea, early design or an ADU review may need to come before a fixed permit-plan price.

• New detached structure
• Garage or accessory-building conversion
• Addition or attached ADU
• Interior conversion or junior ADU
• Custom design or adapting an existing design

Engineers and other specialists still need coordination

Structural, civil, energy, survey, soils, septic, fire-sprinkler, solar, utility, or other consultant work may be required depending on the property and reviewing agency. The client selects, contracts with, and pays each required consultant directly.

A proposal should still state who requests proposals, exchanges project information, tracks listed deliverables, and assembles the permit package. A lower drafting fee may include less administrative coordination.

Revision, submission, and correction support are real work

A proposal should state the included homeowner design rounds and separate them from city or county corrections. It should also explain who prepares the application, submits the drawings, tracks status, responds to agency questions, coordinates specialist revisions, and resubmits the package.

Those responsibilities can remain with the owner, be shared, or be carried by the design team within defined limits. More support generally costs more because it transfers time, communication, and process responsibility—not because the drawings have become physically larger.

• Number and timing of owner design revisions
• Included drawing and specialist coordination
• Submission and application responsibilities
• Status updates and agency communication
• Number of included plan-check response rounds
• Treatment of owner changes after approval

Professional fees are not the whole project budget

Plan-review, permit, school, utility, impact, and other agency fees may apply. Separate specialist, survey, testing, and construction costs can also be significant. A permit-plan proposal should state which fees are included, which are estimates, and which the homeowner pays directly.

California law limits or regulates some ADU fees in certain circumstances, while local agencies maintain their own current schedules and procedures. A general internet estimate is not a substitute for checking the current city or county fees and project facts.

Compare proposals by responsibility and exclusions

When two fees differ, place the proposals side by side and compare what each one includes. Look for the drawing list, design rounds, specialist tasks, submission work, correction support, site verification, extra expenses, schedule assumptions, and conditions that trigger additional fees.

The best proposal is not automatically the least expensive or the one with the longest list. It is the one that matches how much responsibility the homeowner wants to carry and states the limits before work begins.

• What will I receive?
• Which existing conditions are confirmed or assumed?
• Who identifies, retains, pays, and coordinates each required consultant?
• Who submits and follows up?
• How many owner revisions and correction rounds are included?
• Which agency, specialist, and construction costs remain separate?