Who makes this
My name is Joseph Lawrence. I build small, single-purpose calculators for decisions where the arithmetic is annoying and everyone offering to help you has a stake in the answer.
What I am not
I am not a licensed appraiser, a property tax consultant, a contractor, an architect, an electrician or an electrical engineer. I hold no credentials in any of those fields and I am not going to imply otherwise.
What I do instead is show the work. Every calculation on this site names the rule it applies, uses figures you enter from your own documents, and publishes its limitations next to its result. If you need someone to represent you at an appeal hearing, hire a licensed consultant. If you need to know whether your panel can take a charger, hire a licensed electrician — and hand them the worksheet this site prints, because a worksheet is a better thing to argue about than a conclusion.
Where the methods come from
Property tax appeal. The dollars-per-square-foot comparison is the standard evidence residential appeals boards work from. Assessment ratios, mill rates, deadlines and board names are set by state statute and local practice, are shown for orientation only, and are every one of them overridable. The date on your own notice governs.
ADU feasibility. The zoning pre-check summarises California’s statewide ADU baseline in Government Code §§66310–66342 — the 800 square foot and four-foot setback floor, the impact fee exemption below 750 square feet, the parking waiver near transit. A city can be more permissive than that baseline but not more restrictive. Outside California none of it applies and the checklist is a prompt for your own ordinance rather than an answer. Cost defaults are typical California metro figures and are not a quote.
Panel headroom. NEC 220.83, the Optional Calculation for an Existing Dwelling Unit, with 220.57 for electric vehicle supply equipment, 220.60 for non-coincident loads and 625.42 for automatic load management. Section numbers as published in the 2020 and 2023 editions of NFPA 70. States adopt different editions and many jurisdictions amend them, so a calculation that is correct as printed can still be refused locally. The tool says so, on the page, every time.
Everything carries a review date. Where a number depends on your jurisdiction, the tool says so rather than guessing.
Why these three tools and not others
Because each has an incumbent whose interest runs against yours, and in all three cases that shows up in the arithmetic. Appeal firms gate the calculator behind your address so they can sell you a 40% contingency. ADU calculators are published by ADU builders, and every one of them omits the property tax reassessment. Panel calculators overwhelmingly run NEC 220.82, the new-construction method, which produces a larger number than a retrofit requires — and a larger number sells a service upgrade.
None of that requires bad faith on anyone’s part. It just means the free tools available to you are built by people who are paid more when you decide one way.
How this is paid for
Display advertising, and nothing else on this page today. I do not sell leads, take referral fees from appeal firms, ADU builders, electricians or lenders, or receive anything based on which way you decide. If that ever changes it will be disclosed on the page it affects, and it will never sit on the decision a calculator advises.
Corrections
If something here is wrong, tell me and I will fix it. Code sections, statutory citations and filing deadlines especially — those change, and a stale one is worse than none. Contact.