Modular ADU

Modular ADUs are built in a factory and transported to the site, differing from manufactured homes which are built on a permanent chassis.

The total cost of a modular ADU is often comparable to a stick-built ADU once site prep, transportation, and utility connections are factored in.

Factory build times for modular ADUs currently average 4-6 months, subject to supply and demand.

Permitting for modular ADUs still takes an average of 3-4 months, even with pre-approved plans.

Modular ADUs must meet the same local and state building codes as traditional stick-built homes.

The rest of it, in plain terms

• Transportation limits typically restrict modular ADU widths to 8 feet 6 inches without special permits, or up to 15 feet with permits.
• Site preparation, including foundation and utility trenching, must be completed before the modular unit arrives.
• Modular ADUs are often built with reinforced structures to withstand the stress of highway transportation and craning.
• Financing a modular ADU can be more challenging than a stick-built unit, often requiring larger upfront deposits.
• In California, modular ADUs can be up to 1,200 square feet and must maintain a 4-foot setback from side and rear property lines.

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.

• Base unit pricing: $95,000 to $195,000 (excludes site work and delivery).
• All-in estimated costs (California): $200,000 to $360,000+ for larger units.
• Cost per square foot (San Diego): $375 to $600+ for turnkey construction.
• Florida all-in costs: $200,000 to $350,000 for a 1,000 sq ft detached ADU.
• Florida average cost per square foot: $220 to $250 for a mid-grade build.
• Transportation and craning fees: Highly variable based on distance and site access, often adding significant costs not included in the base price.

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.

• California AB 68, AB 881, SB 9: Removes barriers to ADU development, eliminates owner-occupancy requirements, and standardizes regulations across the state.
• Florida Statute 163.31771: Encourages local governments to adopt ordinances allowing ADUs in single-family residential areas to increase affordable housing.
• Florida Building Code: Every modular home in Florida must meet the same building codes as traditional homes, including HVHZ (High-Velocity Hurricane Zone) requirements where applicable.

These words get used interchangeably in marketing and they are legally distinct, which matters for financing, appraisal and resale. A modular ADU is built to the same state or local building code as a site-built house, inspected in the factory under a state program, and set on a permanent foundation. For appraisal and lending purposes it is real property, treated the same as site-built.

A manufactured home is built to the federal HUD code rather than the local building code, carries a HUD label, and is a different product legally. Some jurisdictions permit manufactured units as ADUs and many do not. Financing terms differ and appraisal treatment differs. This is the distinction most often blurred in advertising, and getting it wrong after purchase is expensive to unwind.

The practical advantage of modular is schedule compression and weather independence. Factory work proceeds while site work and permitting proceed in parallel, and the building is never exposed to rain during framing. The practical disadvantage is that the design is constrained by transport dimensions and by what the factory tools, and the crane set introduces a site access requirement that site-built construction does not have.

Modular versus manufactured versus site-built

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.