Detached ADU Builders: How to Compare Contractors' Proposals

Why Detached ADU Bids Aren't Comparable on Price Alone

Two bids for the same detached ADU can carry identical headline prices and describe two entirely different projects. That's because detached units are the most expensive ADU type to build — they carry the full cost of a foundation, framing, roof, and separate utility connections, which is why they sit at the top of the pricing range rather than borrowing structure from the main house.

The numbers behind California detached ADUs are wide, and site work explains much of the spread. In Los Angeles, detached units typically run $150,000 to $400,000, with hard costs of roughly $300–$450 per square foot in most CA metros. Meanwhile, site preparation and utility connections — the sewer lateral, trenching, panel upgrades, grading — can range from about $15,000 to $70,000 depending on the source and lot conditions. A bid that excludes those items can look tens of thousands of dollars lower than a bid that prices them in.

The problem is that bids diverge most on what's excluded. Red-flag scope language shows up consistently across contractor proposals: "by others," "not included," "as required," and "TBD" next to sewer connection, trenching, panel upgrades, grading, or debris hauling. Unclear scope rarely stays unclear — it becomes a change-order discussion after you've signed.

Consider how two $300,000 bids can differ. One contractor quotes only the construction scope; another quotes a full budget that includes roughly $40,000 of site prep and utilities, design, and permits. Without asking each bidder to identify included work, area measurement, taxes, allowances, and exclusions, you're comparing numbers with different denominators. Allowances add another layer: two bids can carry the same allowance dollar amount while assuming very different grades of cabinets, tile, and fixtures — and a low allowance makes a bid look artificially inexpensive.

Site conditions are where detached ADU pricing gets genuinely unpredictable. A constrained hillside lot, a flat accessible lot, and a lot with an existing garage present different work — slope alone can increase labor costs by 10–20%. This is why a credible bid follows a site visit: a builder who hasn't inspected your sewer cleanout, electrical panel, and access path cannot price those items. Unresolved conditions, like unknown utility capacity, should appear in writing as unresolved scope — not disappear behind a fixed-looking headline price.

Before comparing bids, check the assumptions each one carries:

• Whether sewer connection, trenching, panel upgrades, grading, and debris hauling are included or marked "by owner"
• Allowance dollar amounts, whether they cover material only or material plus installation, and how overages are billed
• Whether the price reflects a site visit or generic assumptions
• Which items appear as "TBD," "by others," or "not included"

The takeaway: the bottom-line price is the least reliable number in any ADU proposal. Homeowners in Altadena can start by using the free CSLB license-lookup tool on Altadena Builder Directory to verify any California contractor before requesting bids — every listing in the registry is license-verified against the California Contractors State License Board, so you know the contractors you're inviting to bid are legitimate before you start comparing their scopes.

The Site-Work Trap: Where Detached ADU Proposals Hide Costs

The lowest bid on your detached ADU is often the least complete one — and the gap hides in the dirt, not the drywall. Detached ADUs sit at the top of the cost range because they carry the full price of a foundation, framing, roof, and separate utility connections, according to ADU cost calculators. That site work is exactly where competing proposals diverge.

Watch for five line items that make bids artificially low: sewer lateral connection, trenching for water, gas, and electrical, panel upgrades, grading and demolition, and debris hauling. When any of these appear next to "by others," "by owner," "not included," or "TBD", treat it as a red flag — as ADU builder vetting guides make clear, unclear scope often becomes a change-order discussion later.

A credible bid must follow a site visit. A builder who hasn't inspected your sewer cleanout, electrical panel, and access path simply cannot price those items. Slope complicates utility connections and soil stability, adding 10–20% to labor costs on hillside lots, per Bay Area cost research. If a bidder skips the walkthrough and still quotes a firm number, the unresolved conditions are hiding behind a fixed-looking price.

Benchmark site-work allowances against typical ranges before assuming any bid is generous or padded. Foundation and site work commonly run $15,000–$40,000 depending on soil conditions, while basic site prep runs $10,000–$25,000 and utility connections $15,000–$30,000, per contractor-published cost data. A bid carrying $8,000 for all site work and utilities isn't a bargain — it's an exclusion wearing a number.

• Sewer lateral and connection — the single most common exclusion
• Trenching for water, gas, and electrical lines to the street or main house
• Electrical panel upgrades, typically $4,000–$12,000
• Grading, retaining, and demolition of existing structures
• Debris hauling and site restoration

Prefab "turnkey" pricing hides the same trap. Base units advertised at $90,000–$180,000 often land close to stick-built costs once shipping, crane, foundation, and utility connections are added. If the answer to "who does the foundation and utilities?" is "your contractor," the brochure price buys the unit — not a finished ADU.

Before you level bids, verify who you're leveling. The Altadena Builder Directory lists ADU builders with licenses verified against CSLB data, so you can confirm a Class B General Building contractor — not a B-2 remodeler who can't legally contract structural work — is pricing that site work in the first place.

How to Level Bids: Allowances, Schedules, and Material Quality

Two bids can differ by $40,000 and still describe the same ADU — or differ by $5,000 and describe completely different houses. The bottom-line price is the least reliable number on any proposal, because what each builder quietly excludes matters far more than what they promise.

The fix is a written bid-leveling process. Obtain at least three itemized bids, then issue the same clarification questions to every contractor and document each response. Ask each bidder to identify included work, how square footage is measured, whether taxes are in the price, allowance amounts, and exclusions. Only after normalizing those differences should you compare an "adjusted" price rather than the headline figure — and be wary of any bid that comes in significantly lower than the others, which often signals corner-cutting or future change orders, per ADU contractor guidance.

Schedules deserve the same scrutiny. California's contractor requirements under B&P §7159 call for an approximate start date, a definition of substantial commencement, and an approximate completion date — "we'll start as soon as permits are in" is not a schedule. Remember that the state's 60-day approval clock starts only when the city has a complete application, and getting the application complete is the builder's job, not yours. Compare start dates, overall duration, milestones, and long-lead procurement assumptions across bids.

Allowances are where low bids hide. A skimpy cabinet or flooring allowance makes a proposal look artificially cheap, and two builders can carry identical allowance dollars while assuming very different product grades. For each allowance — cabinets, tile, fixtures, appliances, flooring — require three specifics:

• The exact dollar amount allocated
• Whether it covers material only or material plus installation
• How overages will be billed

Finally, run a material equivalency check. Products described in similar language can differ in manufacturer, product line, grade, thickness, finish, warranty, and service life — a lower-cost substitution may carry different durability, maintenance needs, or energy performance, as construction procurement guidance notes. Ask each bidder to name specific products or provide spec sheets so you're comparing like with like.

Before you request bids at all, verify licenses. Every listing in the Altadena Builder Directory is verified against the California Contractors State License Board, and a free CSLB lookup tool lets you check any California contractor yourself — a step worth taking before three proposals land on your kitchen table.

Verify Before You Sign: Licensing, Contract Legality, and Budget Reserves

The lowest bid in the pile can belong to a contractor who legally cannot build your detached ADU. Before you sign anything, qualification checks matter more than price — and California law gives you clear lines to verify.

Start with the license. A detached ADU involves new footings, foundations, and structural framing, and under California B&P §7057.5, a B-2 Residential Remodeling contractor cannot contract that structural work — only a Class B General Building license covers a ground-up detached unit. Run the license number through the CSLB lookup before you invest hours in bid comparison. Homeowners can use the free license-verification tool in the Altadena Builder Directory, where every listing is checked against CSLB records and confirmed matches display a verification seal.

Then pressure-test the contract itself. California law caps the down payment at $1,000 or 10% of the contract price, whichever is less, and payments cannot exceed the value of work already performed. The contract should also disclose subcontractor use — and treat any request that you pull the permit as an owner-builder a serious red flag, given the employer liability and insurance exposure it transfers to you.

Insurance and references close the loop. Confirm general liability and workers' comp coverage, then ask for a portfolio of completed ADUs and call those references directly. This is where specialization shows: as one California ADU guide puts it, "a company with strong kitchen reviews may never have taken an ADU through plan check." Remodeling skill and ADU plan-check experience are different credentials, and only the second one gets your project through the city.

Finally, protect yourself financially with a contingency reserve. Multiple sources recommend holding roughly 10% of your budget — about $15,000–$25,000 on a typical project, or $30,000 on a $300,000 build — for unresolved site conditions and change orders. Keep that reserve separate from the contract price, and hold optional upgrades like glazing upgrades or landscape restoration as separately priced alternates so they never cannibalize money you may need for structural or utility surprises.

A quick qualification checklist before you sign:

• Confirm an active Class B General Building license through the CSLB lookup — B-2 remodelers are excluded from detached ADU structural work
• Verify the down payment respects the $1,000/10% cap and that progress payments track work performed
• Request certificates for general liability and workers' comp insurance
• Call ADU-specific references, not just general remodeling clients
• Set aside a 10% contingency reserve before comparing bids

One practical note: the CSLB record also confirms the $25,000 contractor bond required by law — a backstop worth knowing about before, not after, you sign.

Your Altadena Bid-Comparison Checklist

You've done the hard analysis — now turn it into a repeatable process. A disciplined checklist keeps you from falling for the lowest headline number, which research consistently shows is the least reliable way to compare ADU bids. Here's how to run the comparison from start to finish.

Step 1: Verify every builder's license first. Before you talk scope or price, confirm each candidate holds an active Class B General Building license — B-2 Residential Remodeling contractors legally cannot contract the structural work a detached ADU requires, per B&P §7057.5 (Green Design and Build's licensing guide). The Altadena Builder Directory's free CSLB lookup tool lets you check any California contractor in minutes; listings in the registry carry a verification seal once a license match is confirmed against CSLB data.

Step 2: Request bids only from license-verified builders, and insist on a site visit. A credible ADU bid follows an inspection of your sewer cleanout, electrical panel, and access path — a builder who hasn't walked the lot can't price those items (per California statute-cited guidance). Collect at least three itemized bids; be wary of bids significantly lower than the others, which often signal corner-cutting or future change orders (ADU vetting experts recommend).

Step 3: Run your bid-leveling sheet. Issue identical clarification questions to every bidder and compare an "adjusted" price after normalizing scope, allowances, and material quality (procurement best practice). Your sheet should capture:

• Site work and utilities — flag "by owner," "by others," or "TBD" next to sewer connection, trenching, panel upgrades, or grading; these items commonly run $5,000–$25,000 for site prep and $5,000–$20,000 for utility connections (per ADU cost data).
• Allowances — the dollar amount, whether they cover material only or material plus installation, and how overages are billed.
• Schedule — an approximate start date, definition of substantial commencement, and approximate completion date, as required by B&P §7159.
• A 10% contingency — roughly $15,000–$25,000 on a typical detached ADU budget (cost-planning guidance).

Step 4: Compare proposals side by side from verified builders. The Altadena Builder Directory's registry — where every listing is license-verified against CSLB — gives you a shortlist you can trust, with ratings drawn only from real, attributed public sources and paid placements clearly labeled. Start there, run your leveling sheet, and let the adjusted numbers — not the headline price — make your decision.