Start with a complete project budget.
Square footage is useful, but it cannot describe access, existing conditions, service work, or your finish decisions. A detached backyard residence and a garage conversion may have similar living areas with very different construction tasks.
Before requesting a final number, define the proposed project type, size, plan status, site conditions, and finish expectations. If some of those items are unknown, identify them as assumptions. A preliminary estimate should tell you what would cause it to change.
Separate these cost categories.
| Budget category | What to clarify |
|---|---|
| Design & professional services | Plans, engineering, site information, revisions, and consultant responsibilities. |
| City review & permits | Applicable fees and who confirms and pays them. |
| Building construction | Structure, enclosure, interior finishes, fixtures, and equipment. |
| Site & utility work | Access preparation, connections, upgrades, trenching, and provider requirements. |
| Restoration | Paving, fences, landscape areas, and existing-home surfaces affected by work. |
| Allowances & contingency | Unselected items versus a reserve for project uncertainty. |
Not every proposal groups the work this way. The point is to locate every category somewhere in the overall budget and identify who owns it.
What affects ADU construction cost in Costa Mesa?
Access: a narrow route can affect handling and equipment choices. Existing conditions: a conversion may reveal work needed in the slab, framing, or enclosure. Utilities: service routes and required upgrades can change the scope outside the dwelling.
Design: kitchens, bathrooms, building form, and structural decisions matter alongside area. Selections: windows, cabinetry, fixtures, flooring, and equipment should match the allowance used in pricing. Interfaces: attached construction can create additional work in the main home.
Compare proposals line by line.
Ask each bidder to identify the same drawing version and describe assumptions in writing. Check which items are fixed scope, which are allowances, and which are excluded. Compare responsibilities for design, review comments, inspection coordination, and utility work.
- Is the price based on the same plans and finish level?
- Which fees or professional services are outside the contract?
- What service and site assumptions support the price?
- How are unforeseen conditions and owner changes handled?
- What restoration and handoff work is included?
Budget decisions also affect the schedule.
Late selections can interrupt purchasing and trade coordination. A design change can require revised drawings and further review. Discuss decision deadlines alongside the construction sequence and record the effect of changes before committing to them.
Use our ADU planning and building process to see when the budget should be updated. For the first conversation, complete a ADU project brief and review the specific scope of detached construction or a garage conversion.