Modular or Site-Built ADU: When a Factory Unit Won’t Work on Your Lot

If you have spent a few weekends reading about modular ADU vs site built, the pitch you keep hearing is about speed and price. That is rarely the thing that decides it on a real lot in Monterey County.

Here is my position after working these projects from Salinas to Seaside to Carmel Valley. The honest comparison is not which method is better. It is which lots a factory unit can physically serve and which scopes a factory catalog simply does not sell.

A module dropped into a flat, open backyard is a reasonable option and I will say so plainly. This article covers the three places that path breaks down: your scope, your access, and the site work hiding under the quoted number.

The Catalog Is the First Filter in a Modular ADU vs Site Built Decision

Most factory ADU companies sell from a short menu. Four or five preset models is typical, usually a studio, a one bedroom, and maybe a two bedroom in fixed footprints and fixed widths.

That works fine if your project matches one of those boxes. If it does not, there is no version of the conversation where it eventually does.

Scopes a factory catalog generally cannot serve:

• Garage conversions, attached or detached
• Attached units that tie into the existing house structure
• Legalizing a structure already standing in the backyard
• Custom footprints driven by setbacks, like a narrow 225 square foot two story shape
• An ADU bundled with kitchen or bath work on the main house
• Lots with historic or coastal overlays that force design changes

These are not hypotheticals. Over one recent summer, the requests that came through our form included a two story garage conversion with an attached unit on a property that sits on the historic registry and inside the coastal zone, a homeowner who wanted an existing backyard studio removed and replaced while the main house got remodeled, and a straightforward 450 square foot detached unit.

Only that last one is a catalog project. If your home carries a historic listing, start with what that designation actually changes before you shop any builder.

Stand in Your Driveway and Measure the Path

A module is built 300 miles away and has to arrive intact. Delivery gets decided by your street, your side yard, your slope, your trees, and the wires overhead. The floor plan has nothing to do with it.

Walk out and check these before you fall in love with a model:

• Clear width of the access path from street to backyard, fence post to fence post, not gate to gate
• Overhead lines, both the service drop to your house and anything crossing the street
• Tree limbs and trunks that would have to come out, and whether you want them out
• Where a crane could set up with outriggers down, usually the street or driveway
• Distance the module would have to travel over soil, which in January is soaked
• Slope. A rear yard that steps down is a problem long before design is
• Turning radius on narrow older streets in Salinas, Pacific Grove, and parts of Seaside

A crane can fly a module over a house in some cases. That takes clear staging room, a lane closure in many spots, and clean separation from utility lines.

If your rear yard is landlocked behind the house with a four foot side yard on each side, delivery is off the table. That answer comes before any design question. A proper site walk before anyone prices the job is where this gets settled, and it should take an hour, not a sales appointment.

The Quoted Price Is for the Box, Not the Project

This is where Central Coast projects separate from the national averages you read online. A factory number is a delivered unit number. The work underneath it is yours either way.

Site costs that exist whether you go modular or site built:

• Foundation or slab, engineered for your soil
• Grading and drainage so winter water goes somewhere other than the new slab
• Trenching for water and sewer, or a septic evaluation and possible system upgrade
• Electrical service, panel capacity, and the meter set
• Agency fees across planning, building, and environmental health
• On parts of the Peninsula, a water permit that comes before the building permit

Costs move with the lot and the finish level, so treat every figure as a range. Our own ADU estimator puts a 600 square foot detached unit at a standard finish somewhere in the neighborhood of $463 to $608 per square foot with a typical four to eight month timeline, and that spread exists because site conditions genuinely swing it.

One Salinas homeowner asked us in writing whether an ADU could be built for around $40,000. I understand where that number comes from, and I would rather say plainly that it does not cover the site work here than let someone plan around it.

The only honest figure comes from a site visit and a written scope from whoever will actually build it. If you are holding bids that are far apart, compare what each one includes underground, not what the finished unit looks like.

What Your Lot and Scope Point Toward

This is the quick read I give homeowners on the first walk. It is a direction, not a verdict, and a site visit can change it.

The Five Minute Driveway Check

Before you request a single factory quote, walk the path a module would travel and answer these five questions.

The Delay Nobody Prices Into Either Method

A module can be set in a single day and still sit dark for weeks. Utility scheduling is the most repeated real world delay I see, and no builder controls it.

What a contractor does control: crew scheduling, trade sequencing, inspection requests, and keeping the site clean and safe between phases.

What nobody controls: meter sets, service energization windows, and the utility’s own calendar.

A homeowner we converted a garage for in Salinas put it this way in her review: “The project took a little longer than expected, but this was due to PG&E’s scheduling and availability.” She still hired us for the next project, and I appreciate that she named the real cause.

Ask any builder, factory or site, how they sequence the service application and when they submit it. A builder who applies early and tracks it is doing the part they can do.

Who Each Method Actually Fits

I want to be fair to the modular path, because it is a legitimate one. The California Department of Housing and Community Development ADU Handbook lists factory built and manufactured units as a recognized option and notes they can shorten construction time.

Factory built makes sense when: the lot is flat with open access, the layout is a standard studio or one bedroom, there is no existing structure in the mix, and you want fewer weeks with the yard torn up.

Site built makes sense when: you are converting a garage, attaching to the house, working a constrained or sloped lot, legalizing something already standing, drawing a custom footprint, or pairing the ADU with main house work.

Season matters too. Forecasters have been calling for a strong El Nino pattern through this fall and into early spring, which means more weather exposure for anything framed outdoors. If you are weighing a start now against waiting, that argument favors less time with an open frame, and it is worth a conversation about whether an existing backyard structure can carry the project instead.

One more thing people miss. A factory unit still needs local permits, local inspections, and local utility work. Requirements vary by jurisdiction, so confirm current rules with your city or county agency before you commit to either path. If you are still early, comparing pre-approved plan sets against custom drawings is a separate decision worth understanding on its own.

Frequently Asked Questions About Modular and Site-Built ADUs