What Is ADU Feasibility?

Key takeaways

• ADU feasibility asks one thing: can this lot take an ADU, and which kind?
• Check it before you pay for full drawings.
• Work in order: zoning, overlays, lot shape, type and size, utilities, access, budget.
• In California, the lot must allow homes and have a home on it or proposed ¹.
• California cities must allow at least an 800 sq ft ADU with 4-foot side and rear setbacks ⁵.
• A feasibility check is not a permit. Only the city approves an ADU.

What ADU feasibility means, in plain English

An ADU is an accessory dwelling unit: a small second home on the same lot as a house.

"Feasible" just means "can be done". So ADU feasibility asks whether your lot can hold one. It asks it in four ways: is it legal, does it fit, can it get water, sewer and power, and can you pay for it?

A feasibility study is the homework that answers those four. It can be a free online check, a paid report from a designer, or a builder’s site visit.

Why feasibility matters to you

Design fees are paid before you know the build price. If the lot has a hidden problem, that money can be wasted.

Common late surprises are an easement, a slope, a septic system, or a sewer line that is too far away. An easement is a recorded right for someone else, such as a utility, to use part of your land.

A short check up front costs less than redrawing plans later.

Feasibility is a yes, a no, or a "yes, if"

Most lots in California can legally hold some kind of ADU. The real question is which kind, how large, and at what cost to make work.

A feasibility check turns that into an answer you can act on: build a 750 sq ft detached unit at the back, convert the garage, or stop because the sewer cannot reach.

Feasibility is mostly about ruling things out early. A slope, an easement, a septic system or a historic overlay found after drawings are done is the expensive way to learn it.

The guide below turns common findings into the next step.

What a feasibility check covers, in order

Work from the hardest constraint to the softest. A fail at step 1 makes the rest moot.

1. Zoning. In California the lot must be zoned to allow single-family or multifamily residential use and include an existing or proposed dwelling ¹. Elsewhere, check whether the city allows ADUs at all; see ADU laws by state.
2. Overlays and hazards. Coastal zone, flood zone, fire hazard area, historic district. See ADUs in the coastal zone and ADUs in flood zones.
3. Lot geometry. Width, depth, setbacks, easements, slopes and trees decide where a unit can sit. California caps side and rear setbacks for new ADUs at four feet ¹.
4. Type and size. Pick the form the lot allows: detached, attached, a garage conversion or a junior ADU, then the size limit that applies.
5. Utilities. Sewer or septic, water, electrical capacity, gas if any. Where a private sewage system is used, approval by the local health officer may be required ¹; see ADUs on septic systems.
6. Access. How workers, materials and future tenants reach the unit. California says no passageway may be required for an ADU ¹, but construction access is a practical cost.
7. Budget. Fees, design, construction and utility work against what you can finance. Use cost drivers, not a single price; see ADU cost.

A check that covers all seven gives you a short list of buildable options to take to a designer.

What a feasibility study can and cannot tell you

Why feasibility is narrower in California

California removes much of the guesswork. ADU applications must be approved ministerially, without discretionary review or a hearing ³, and a local ordinance that does not comply with state law is null and void, with state standards applying instead ².

For a single-family lot, § 66323(a) requires approval of an ADU within existing space plus a junior ADU, and of one new detached ADU with four-foot side and rear setbacks ⁴.

So in California the feasibility question is rarely "is any ADU allowed?" It is "which of the protected options fits this site and budget, and is anything bigger worth the local process?" Once you apply, the permitting agency must say whether the application is complete within 15 business days and decide within 60 days of a complete application where a home already exists on the lot ³. Current timing data is on ADU permit times by city.

Sources: [2] [3] [4]

Run your own check

• Feasibility checkerZoning, lot and utility basics
• Size envelope calculator
• ADU type selector
• ADU planning hub
• Timeline planner

Kinds of feasibility checks

Worked example: a quick check on one lot

A hypothetical California lot, for illustration only. It is 50 feet wide and 125 feet deep, with one house and a two-car garage at the back.

Result: "yes, if". The garage conversion keeps the trench shorter if the garage already has plumbing nearby. The new detached unit gives more space. Price both before picking one.

California size and height floors to know

Government Code 66321 lets a city set size limits for ADUs, but with floors ⁵. A city may not set a maximum below 850 sq ft of interior living space. For an ADU with more than one bedroom, the floor is 1,000 sq ft ⁵.

A city also may not use lot coverage, floor area or front setback rules in a way that blocks an ADU of at least 800 sq ft with 4-foot side and rear setbacks ⁵.

Height has floors too. A detached ADU on a lot with a home must be allowed at least 16 feet. Near a major transit stop, it is at least 18 feet, plus up to 2 more feet for a roof pitch that matches the main house ⁵.

These are California rules. Other states vary widely. See ADU laws by state.

Sources: [5]

Where feasibility findings show up later

• Site plan: setbacks, easements and the unit’s footprint. See ADU site plan.
• Permit set: the type and size you chose.
• Utility letters: sewer, water and power capacity. See ADU utility connection.
• Builder bids: site work, trenching and access costs.
• Loan offer: the budget your lender reviews. See ADU financing.

What drives the cost of a feasibility check

HyreADU does not publish a typical price for a study. We have no dated, sourced data on it that meets our standard.

What changes the cost: whether a site visit is included, whether a survey is needed, how many layouts are tested, and whether a rough build budget is part of it.

Some design-build firms fold the check into a design agreement. Ask whether the fee is credited if you go ahead. See ADU design-build.

Common misreadings

Myth A feasibility report means the city will approve my ADU.

Reality No. Only the permitting agency decides, after you apply. A report lowers the risk; it does not remove it.

Myth If my lot is small, an ADU is not feasible in California.

Reality Not always. Cities may not apply some rules in a way that blocks an 800 sq ft ADU with 4-foot side and rear setbacks ⁵. A garage conversion may also need no setback ¹.

Myth The city’s own ADU rules are always the last word.

Reality Not in California. A local rule that breaks state ADU law is void, and state standards apply instead ².

What to ask whoever runs your feasibility check

• Which zoning district is my lot in, and what overlays apply?
• Where are my true lot lines? Do I need a survey?
• Are there recorded easements on title?
• Can the sewer or septic take another unit?
• Is my electrical service big enough, or will it need an upgrade?
• Which ADU types fit, and how big can each be?
• What could still go wrong after this check?

Checklist: run your own ADU feasibility check

1. Look up your zoning district on the city or county map.
2. Check for coastal, flood, fire and historic overlays.
3. Get a copy of your title report to spot easements.
4. Sketch the lot and draw setback lines inside it. See setback.
5. Pick the ADU types that fit: conversion, attached or detached.
6. Find where sewer, water and power reach the lot.
7. Run the size envelope calculator for each type.
8. Set a rough budget range, then compare it with your financing.

Related terms

Accessory Dwelling Unit

An accessory dwelling unit (ADU) is a second, self-contained home on a lot that already has, or will have, a main house or apartment building, with its own space to live, sleep, cook, eat and wash.

ADU Zoning

The local zoning rules that say where an ADU may go on a lot and how big and tall it may be. In California, state law requires cities to allow ADUs on lots zoned for residential use and limits how far local zoning can restrict them.

ADU Size

ADU size is the floor area of an accessory dwelling unit, usually counted as interior livable square feet, and it is the number that state and local rules cap, protect and use to set fees.

Setback

A setback is the minimum distance a zoning code requires between a building and a property line, measured horizontally from the line to the nearest wall or other regulated part of the structure.

ADU Utility Connection

An ADU utility connection is the way an accessory dwelling unit gets water, sewer, electricity and, where used, gas: either as a branch off the main house's existing service, or as a new, separate connection running directly from the utility to the unit.

Ministerial Approval

Ministerial approval is a permit decision in which an official checks an application against fixed, objective standards and must approve it if they are met, with no public hearing and no discretion to deny it on judgment.

ADU Permit

An ADU permit is the building permit a city or county issues to allow construction of an accessory dwelling unit, after checking the plans against local zoning standards and the building code.

Buildable Area

Buildable area is the part of a lot where a building may legally and physically go, found by subtracting setbacks, easements and other no-build areas from the whole parcel.

Also referenced from: Accessory Dwelling Unit, Minimum Lot Size, Buildable Area, ADU Parking Requirement, ADU Site Preparation