How we estimate ADU costs (the calculator methodology)

An online calculator is useful only when it exposes its assumptions. This method organizes user-entered project type, size and site conditions into a planning comparison; it does not claim a proprietary completed-project dataset or replace an itemized proposal. Use the result to identify missing questions, then test it against the cost breakdown, the responsible agency's current fees and written bids from properly licensed contractors.

Inputs we actually use

• —City — identifies the permit authority and the local fee schedule that must be checked; it is not a hidden price multiplier.
• —Build type — new detached, attached, garage conversion, basement conversion, second-story addition.
• —Square footage — changes quantities while fixed-scope items such as a kitchen, bath, consultants and utility work do not scale in a simple straight line.
• —Finish tier — standard, mid, designer; primarily adjusts cabinetry, flooring, and fixtures.
• —Site complexity — flat, sloped, hillside, hand-dig required.

Anchors

The planning range is organized around scope categories rather than an asserted market average: design and consultants, agency charges, site and utility work, structure and enclosure, systems, finishes, restoration and contingency. Reusing an existing structure can avoid some new work but can also reveal repair, code and coordination needs. Verify every allowance against the property's actual condition, the responsible agency's current fee pages and written licensed-contractor bids.

What the calculator does NOT do

It does not quote your project or verify parcel eligibility. A defensible project number requires drawings and scope, property investigation, current agency charges, utility and site findings, and written trade or contractor pricing. Use the output as a question list with the LA cost anatomy, then compare proposals with the CSLB verification workflow.