Why San Diego ADU Quotes Vary by $100K+ (2026 Guide)

If you've collected bids from two or three ADU builders in San Diego, you've probably had this moment: one quote comes in at $260,000, another at $390,000, and they're both for the "same" detached ADU in your backyard. The natural reaction is to assume the higher one is overpriced — or that the lower one found a way to do it cheaper.

Most of the time, neither is true. The two bids aren't describing the same project. They're describing two different scopes of work that happen to share the same square footage. I write and review these proposals every week, and the gap between a complete bid and a thin one is almost always $80,000 to $120,000 of work that someone has quietly left off the page.

Here's how to read what's actually in front of you, so you can compare quotes on equal footing instead of just comparing the bottom-line number.

The short version: square footage isn't a scope

A 750-square-foot ADU is not a fixed product like a car with a sticker price. Two builders can quote the same size and mean completely different things by it. One number includes design, engineering, permits, fees, site work, and utility connections. The other includes the building shell and a hopeful guess at everything else.

In San Diego, a complete, turnkey detached ADU generally runs $375 to $600+ per square foot, which lands most projects between $300,000 and $450,000 all in once you count design, permits, sitework, and utilities. When a bid comes in well below that range for a ground-up detached unit, the question isn't "how did they do it cheaper?" It's "what did they leave out?"

How we quote it: two agreements, not one guess

Before I get into what's in a bid, here's how we structure the work, because it's the single biggest reason our numbers are built differently from a one-page all-in quote.

We split an ADU into two separate agreements. First, design and permitting. We design the unit, engineer it, and produce the complete set of construction documents — the full plan set that gets submitted to the city. Then we price the build off that finished plan set: a real construction number based on the actual project, not a guess made before it's been drawn.

The reason is simple: you cannot honestly price a build before the construction documents are done. Anyone who hands you a single fixed all-in number on day one is pricing a building that hasn't been drawn or engineered yet. To make that early number look solid, they lean on lowball allowances and assumptions — and those are exactly the things that move later. We price once the plans are complete, and because plan check rarely produces major changes, that number holds through city review instead of unraveling during it.

What a complete ADU project includes

Whether it's quoted in one agreement or two, getting an ADU from dirt to a certificate of occupancy takes all of this. Knowing the full list is how you spot what a thin quote left out:

• Design and architecture — floor plans, elevations, and a design that actually fits your lot and your city's rules. (Phase one for us.)
• Structural engineering and required reports — including a soils or geotechnical report on sloped or hillside lots, which runs roughly $2,500–$4,000 on its own. (Phase one.)
• Permits, plan check, and fees — city submittals, plan-check fees, and any impact or school fees that apply. (Phase one.)
• Site work — grading, demolition, access for equipment, and prepping the pad.
• Utility connections — running water, sewer, gas, and electrical from the main house or street to the new unit, including trenching.
• Electrical service — often a main panel upgrade on older San Diego homes that can't carry a second dwelling.
• The building itself — foundation, framing, roofing, windows, insulation, drywall, HVAC, plumbing, and electrical rough-in and finish.
• Finishes — flooring, cabinets, countertops, fixtures, appliances, and paint.
• Final connections, inspections, and sign-off — getting the unit legally finished and occupiable.

A quote that leaves any of these out isn't necessarily dishonest — but it's incomplete, and the missing pieces don't disappear. They come back as change orders, "allowance overages," or a separate invoice you weren't expecting.

Where the $100,000 actually hides

Low bids rarely lie outright. They get to a lower number by either excluding line items entirely or by setting allowances so low that they're guaranteed to be exceeded. These are the usual suspects:

The allowance trap — and how we handle finishes instead

This is the one that catches the most homeowners. An "allowance" is a placeholder dollar amount for things you haven't picked yet — cabinets, flooring, countertops, appliances. A builder can make a bid look $40,000 cheaper just by writing in allowances no one could actually hit. If a quote allows $4,000 for all the cabinets and countertops in a full kitchen and bath, that number is fiction. You'll pick real materials, the real cost will be double or triple, and the difference becomes your problem mid-project.

We handle finishes a different way: they aren't in our construction price at all. You buy your finish materials — flooring, cabinets, countertops, fixtures, appliances — directly from the supplier, and I pass my contractor discount on to you, so you pay the store at my cost with no markup from us. So you're not flying blind, I give you a separate expectation sheet showing what those products are likely to run for the level of finish you want. Honest, good-quality numbers — not the cheapest thing on the shelf, and not a lowball figure designed to creep.

This is the opposite of the exclusions I warned about above, even though it can look similar at first glance. The difference is transparency. A lowball allowance buries cost inside a number that's guaranteed to balloon into a change-order invoice from your contractor. Our finish carve-out is fully disclosed: you know exactly what's not in our price, you know roughly what it will cost from the expectation sheet, and you capture our discount instead of our markup. Nothing about it surprises you later — and because finishes aren't in our contract, your construction price doesn't move when you change your mind about the tile.

One practical note for comparing bids: some builders fold finishes into a single all-in number, often through a lowball allowance, while we carve them out at cost. To compare fairly, add your expected finish budget from the expectation sheet onto our construction number, then line that up against their all-in figure. Do that and you're comparing real total project cost — and the contractor discount usually lands in your favor.

The fee piece most bids get wrong

San Diego ADU fees changed in ways that cut both directions. Under state law (SB 13), ADUs 750 square feet or smaller are exempt from development impact fees and utility capacity charges — that's roughly $20,000 in savings, and it's a real reason to think carefully about sizing. But the City of San Diego no longer offers a blanket plan-check fee waiver on standard ADUs, and ADUs larger than 750 square feet pay proportional fees. A good bid reflects your actual size and the current fee structure. A sloppy one either ignores fees entirely or copies last year's assumptions.

Why a real number sometimes comes in two parts

Here's the objection I'd raise if I were you: a competitor handed me one number for the whole project, and you're telling me you'll quote the construction later. Isn't one number safer?

It feels safer. It usually isn't. That single upfront figure is priced against a building that hasn't been fully drawn or engineered yet, so it leans on lowball allowances and assumptions to look complete. When the design firms up or real material costs come in higher than the placeholder, the number moves — through change orders, after you've already committed.

We design and permit first, then price the build off the completed construction documents — the same plan set going to the city. You get two real numbers instead of one optimistic one, and after the design-and-permit phase you own a complete set of plans rather than a guess. We're not asking you to commit to the whole project before it's been drawn.

And because design, permitting, and construction stay under one roof, there's no finger-pointing when the build starts — no GC blaming the architect and architect blaming the GC while you stand in the middle paying for the gap.

How to compare two ADU bids the right way

Don't start with the total. Start with the exclusions page — that's where the truth is. Then walk both bids through the same checklist:

• Does it include design and engineering, or is that "by owner"?
• Does it include a soils report if your lot is sloped?
• Are permits and city fees a real cost, or an allowance, or excluded?
• Is site work priced for your lot, or does it assume flat and easy?
• Are utility connections and trenching included?
• Is a panel upgrade included if your home needs one?
• How are finishes handled — a realistic number, a lowball allowance, or carved out at cost?
• At what point does the price actually become fixed: at signing on day one, or once the construction documents are complete?
• What's the change-order process, and what triggers one?

Run both quotes through those questions and the $130,000 gap usually shrinks to $15,000 — and now you're comparing builders, not bid-writing tactics.

Why our number sometimes looks higher — answered straight

We're typically not the cheapest quote a homeowner gets, and I'd rather explain why than pretend otherwise. We don't compress the whole project into one optimistic day-one figure, and we don't bury cost in lowball allowances. We design and permit first, then price the build off the completed construction documents. Finishes aren't marked up inside our number at all — you buy them direct at our contractor discount, with an expectation sheet so you can budget honestly. So when you stack our construction quote plus your finish budget against a competitor's all-in bid, you're usually looking at a similar real total — ours just doesn't arrive as a string of change orders after you've committed, and you keep our discount on the finishes instead of paying someone's markup.

That's the trade-off. If the lowest possible number on day one is the priority, we may not be your builder. If a number you can rely on through the end of the project is the priority, that's exactly what our two-phase process is built to give you.

Related reading: Cost to Build an ADU in San Diego (2026), How Long It Takes to Build an ADU, and How to Finance an ADU in San Diego.

Frequently asked questions

Why are San Diego ADU quotes so different from each other?

Because square footage isn't a standardized scope. One bid may include design, permits, fees, site work, and utilities, while another includes only the building shell and excludes or under-allowances the rest. The gap is usually missing scope, not a cheaper way of building.

What's usually left out of a cheap ADU bid?

The most common exclusions are design and engineering, soils/geotechnical reports, permit and city fees, site work and grading, utility connections and trenching, and electrical panel upgrades. Finishes are often handled with a lowball allowance. These costs don't disappear — they return as change orders later.

How much does a detached ADU actually cost in San Diego in 2026?

A complete, turnkey detached ADU generally runs $375 to $600+ per square foot, which puts most projects between $300,000 and $450,000 all in, including design, permits, site work, and utilities.

What is an allowance, and why does it matter when comparing bids?

An allowance is a placeholder dollar amount for items you haven't selected yet, like cabinets or flooring. Builders can make a bid look cheaper by setting allowances unrealistically low. Always compare them line by line, not just the bottom-line total. At IL Total Design & Build, finishes aren't in our construction price or marked up at all — you buy them directly from the supplier at our contractor discount, and we hand you a written expectation sheet showing what good-quality products are likely to cost, so you can budget with no surprises and no hidden margin.

Do you mark up finish materials?

No. You purchase your finish materials directly from the supplier and pay our contractor discount, not retail and not a marked-up price from us. We provide a separate expectation sheet so you know roughly what those products will cost for the finish level you want. To compare our quote fairly against an all-in bid, add your expected finish budget to our construction number.

Why do you separate permitting and construction into two agreements?

You can't honestly price a build before the construction documents are done. We handle design and permitting first, producing the complete plan set that goes to the city, then price the build off those finished documents. That gives you an accurate construction number instead of a day-one estimate propped up by lowball allowances — and because plan check rarely produces major changes, that number holds through city review.

Are ADU permit and impact fees included in a bid?

They should be, as a real itemized cost. Under SB 13, ADUs 750 square feet or smaller are exempt from impact fees and utility capacity charges in California, saving roughly $20,000. Larger ADUs pay proportional fees, and the City of San Diego no longer waives plan-check fees on standard ADUs.