What will your project actually cost?

How the calculator works

Start with the kind of space you want to create, then choose its square footage, finish level and region. The calculator multiplies a project-type baseline by the selected area and the finish and regional factors. Those factors are budgeting assumptions in this tool, not measured county averages or promises about the price of a particular property. Two homes in the same neighborhood can need quite different work.

The construction subtotal receives a soft-cost allowance of 9% for design, engineering, permits and project management. Selected site add-ons are then added as fixed planning allowances. The displayed low end is 88% of that total and the high end is 114%. Those bounds are a modeling choice, not a statistical confidence interval. Totals and the midpoint are rounded to the nearest $5,000; the displayed cost per square foot is rounded to the nearest $5.

The tool cannot inspect existing framing, measure utility routes or establish a jurisdiction's actual fees. It does not separately calculate financing charges, temporary housing, lost rental income, furnishings or ongoing ownership costs. It also has no separate contingency input. Do not assume that the high end covers every unknown. Review exclusions, allowances and contingency with the written proposal rather than treating this screen as an all-inclusive price.

What drives cost in Los Angeles and Ventura County

Size matters, but cost does not move evenly with each extra square foot. A larger living room adds different work from another bathroom. Finish selections change material and installation scope: cabinetry, tile, counters, windows and fixtures should be compared by specification, not by a label such as “elevated.” Keeping a clear list of selections helps make two proposals comparable.

Site work is another source of uncertainty. A new foundation, demolition, drainage changes or limited access may affect the scope before interior construction begins. On a hillside parcel, the structural and site review should establish what grading, support and access work is necessary. Selecting a hillside allowance here does not confirm that a particular solution is feasible or that its price is covered.

Utilities need their own review. Existing electrical capacity, water service and the route to a sewer connection can affect both design and cost. A garage conversion may reuse a building shell but still need substantial utility or habitability work. An above-garage ADU needs an assessment of the structure below. Avoid adding an allowance twice if the same work is already covered in the base construction scope.

Fees depend on the reviewing agency and the project. A mailing address alone does not establish city versus county jurisdiction. State impact-fee exemptions do not eliminate every permit, school or utility charge. Read our California ADU impact-fee guide, which cites the March 2026 HCD handbook and current statutes, then confirm the applicable fee schedule for your parcel. Local rules and site conditions vary.

Cost per square foot by project type

The table shows this calculator's existing construction baselines for standard finishes and the San Fernando / Conejo Valley setting. They are preliminary model inputs, not bids, minimum charges or independently verified market averages. Finish and region selections, the 9% soft-cost assumption and any chosen site allowances change the final range. The result panel's per-square-foot figure includes those additions, so it is not directly equivalent to this baseline table.

For context beyond the model, compare the ADU build-cost guide, home-addition cost guide and Los Angeles garage-conversion cost guide. Building upward raises different structural questions: see second-story addition costs. For rooms with concentrated plumbing and finish work, read master-suite addition costs and kitchen addition and bump-out costs.

Turn the range into a written proposal

Save the project type, size and selections you used, and write down your priorities. Have your address, any existing plans and photographs available for the property review. Explain whether you want independent living space or more space within the main home; that distinction changes the design questions. Also identify the work you want retained, such as parking, outdoor space or existing rooms.

Feasibility comes before a construction price: the address and zoning review, existing conditions and a conversation about the scope guide design. A written construction proposal follows design and should identify what is included, what is excluded and which selections remain allowances. See HomeAdd's process or request a free property check. Use the calculator to organize that conversation, not to replace it.

Calculator questions