For a factory-built ADU, how close the builder sits matters far less than most people assume. What decides your cost and your feasibility is crane access to the rear of your lot, the width of your street, and who performs the set and the utility connections. This page explains when factory-built is the right answer and when a local site-built crew genuinely is.
ADU construction typically requires a licensed general contractor; in California, this is often a Class B General Building Contractor license.
The average cost to build an ADU ranges from $250 to $500+ per square foot, depending on location, materials, and whether it is custom-built or prefabricated.
Site preparation and utility connections (water, sewer, electrical) are major cost drivers, often adding $10,000 to $30,000+ to the base construction price.
Design-build firms handle both the architectural plans and construction, which can streamline the process and reduce miscommunication between architects and builders.
Permitting timelines vary drastically by municipality, but state laws like California's AB 68 mandate that local agencies ministerially approve compliant ADU applications within 60 days.
Figures below are researched market numbers for budgeting and comparison. They are not quotes. Final pricing depends on your site, your jurisdiction, finish selections, foundation and utility distance.
ADU rules are set locally and change often. These are the provisions that most commonly control the outcome. Verify against your own jurisdiction before you design anything.
For a site-built ADU, local matters enormously: the crew is on your property for months, and a builder two hours away will staff your job with whoever is available. For a factory-built ADU the calculation inverts. The building is manufactured in a controlled plant and delivered complete, so the relevant question is not how close the factory is but who performs the site work, the set and the utility connections, and whether they have done it in your jurisdiction before.
Freight is real but usually smaller than people expect relative to total project cost, and it is predictable. Crane access is the variable that actually decides feasibility. A unit that has to be craned over a house needs a larger crane, a street closure permit in many cities, and sometimes a traffic control plan. That combination can add meaningfully to the budget and it is entirely a function of your lot, not of the builder's location.
The honest version: if your lot has rear-alley access or a wide side yard, factory-built is usually faster and more predictable. If your only approach is over the roofline of an existing house on a narrow urban street, a local site-built crew may genuinely be the better answer, and a builder who tells you that instead of selling you a module is worth listening to.
| Crane or truck access to the rear of the lot | The single biggest feasibility factor. |
| Overhead utility lines along the approach | May require utility coordination or temporary drop. |
| Street width and closure permit requirements | Urban lots often need traffic control. |
| Setback and lot coverage limits | Determines whether a standard module footprint even fits. |
| Local labor availability and cost | Where site-built becomes competitive. |
| Schedule certainty required | Factory build reduces weather exposure. |
| Freight distance from the plant | Predictable, and usually smaller than crane cost. |
| Local inspector familiarity with modular | Smoother approvals where modular is common. |
Most pages on this subject put the cost behind a contact form. The reasoning is understandable and the result is that you cannot budget, cannot compare, and cannot tell whether a bid is reasonable until you have already given up your phone number to three companies.
We publish the figures instead. They are researched market ranges rather than quotes, and site conditions will move them more than any other factor, but a range you can plan against beats a call-back you have to wait for. The constraints that decide feasibility — setbacks, height, utility distance, site access — are stated up front for the same reason. If your project is not viable, that is worth knowing on the first visit rather than the third phone call.
Factory production windows move with queue depth, material lead times and seasonal demand. We quote the current window rather than a fixed promise, because anyone promising a specific delivery date months out without seeing the factory schedule is guessing. Permitting is usually the longer pole: in ministerial states the review clock is defined by statute, while in discretionary jurisdictions it can run considerably longer.
The sequence that controls your schedule is site plan, permit submittal, plan check and corrections, permit issuance, factory slot, delivery and set, then utility connection and final inspection. Factory build time overlaps permitting only if you commit to the slot before permits are issued, which carries its own risk.
Tell us the scope and the ZIP. You get a real delivered-and-installed number, not a "contact us for pricing" runaround.