The Cheapest Way to Build an ADU Without Cutting Corners

The short answer: The cheapest way to build an ADU in California is usually a conversion of space you already own, kept at or below the size thresholds that trigger fees, built from a pre-approved plan with utilities close to the existing connections. One 2026 builder guide puts a garage conversion at $250 to $450 per sq ft against $300 to $550 for a new detached unit, state law exempts ADUs of 750 sq ft or less from impact fees, and a pre-approved plan can bring design fees down to about $2,000 to $5,000. What you should not cut: structural work, fire safety, permits and licensed trades.

Last verified: October 5, 2026 · Written by the ADU Scope editorial team

Lever 1: Convert, the Cheapest Way to Build an ADU

First, reusing an existing slab, walls and roof is the single biggest saving. ADU Scale’s 2026 California ranges show the gap:

Type	Per sq ft	Total project
Junior ADU (JADU)	$200 to $350	$80,000 to $160,000
Garage conversion	$250 to $450	$120,000 to $220,000
Attached ADU	$300 to $500	$200,000 to $320,000
Backyard detached	$300 to $550	$240,000 to $380,000

California law also helps conversions. A city cannot require a separate utility connection for a conversion ADU or a JADU, and those units are not treated as a new residential use when connection fees are calculated unless they are built with a new home. The trade-off is that older garages may need retrofit work, which the same builder puts at $5,000 to $25,000 for pre-1970 structures.

Lever 2: Stay Under the Fee Thresholds

California’s size thresholds are the closest thing to a built-in discount:

• 750 sq ft or less: no impact fees for an ADU (Gov. Code section 66311.5).
• 500 sq ft or less: no impact fees for a JADU. Separately, a unit under 500 sq ft of interior livable space is treated as not increasing assessable space, so no school fee applies.
• Above 750 sq ft: impact fees are charged in proportion to the ADU’s size compared with the main house.

Overall, the difference can be thousands of dollars, and cities set their own permit and plan-check fees on top. Designing at 750 sq ft rather than slightly above can pay for itself, but test it with a written fee estimate from your own city. Our guides to impact fees and permit cost by city explain what applies.

Lever 3: Use Pre-Approved Plans for the Cheapest Way to Build an ADU

For example, design fees can run $10,000 to $25,000 for a standard detached ADU, according to The ADU Pro. A city’s pre-approved plan can bring that to about $2,000 to $5,000, and cities that run these programs usually describe faster review as well. See ADU design cost: architect vs. pre-approved plans for how they work and what they do not do: fit every lot, remove utility or drainage requirements, or allow unlimited customization.

Lever 4: Keep Utilities Short and Shared

Above all, distance is the main driver of utility cost. For instance, at $50 to $150 per linear foot of trench, a 120-foot run is $6,000 to $18,000 before any connection fee. Practical savings, using the contractor ranges in our guide to hidden costs and utility hookups:

1. First, place the ADU as close as practical to the existing sewer, water and electric connections.
2. Second, share the existing sewer lateral if capacity allows: tying in is reported at $3,000 to $8,000 against $10,000 to $30,000 for a new lateral to the main.
3. Third, use a subpanel ($2,000 to $5,000) instead of a new meter ($3,000 to $15,000) where your utility allows it.
4. Finally, consider an all-electric design to avoid a gas line ($1,500 to $5,000), subject to local rules.

Is Prefab the Cheapest Way to Build an ADU?

Not automatically. In fact, ADU Scale lists prefab and modular units at $200 to $400 per sq ft delivered and $180,000 to $320,000 all-in, a range that overlaps the lower half of its $240,000 to $380,000 range for site-built detached units. The factory moves part of the work indoors, but the foundation, crane, site work and utility connections still have to be paid for. Therefore, ask any supplier for the total installed price, not the price of the unit.

Putting Rough Numbers on the Levers

The levers do not add up neatly, because each range comes from a different source and depends on the lot. Overall, the arithmetic below only shows the order of magnitude, using the ranges quoted above.

Lever	Comparison	Rough difference
Convert instead of building detached	Midpoint of garage conversion ($350 per sq ft) against backyard detached ($425 per sq ft)	$75 per sq ft, or about $30,000 on 400 sq ft
Pre-approved plan instead of a standard architect design	$2,000 to $5,000 against $10,000 to $25,000	About $5,000 to $23,000
Tie into the existing sewer lateral instead of a new one	$3,000 to $8,000 against $10,000 to $30,000	About $2,000 to $27,000
Subpanel instead of a new electric meter	$2,000 to $5,000 against $3,000 to $15,000	Up to about $13,000, if your utility allows it

In summary, the widest differences come from conversion and from utility distance, which is why those two decisions deserve the most attention before design starts.

What Not to Cut in the Cheapest Way to Build an ADU

However, some savings are false economies. In our view these are the ones not to touch, and a licensed contractor should confirm the details for your project:

• Permits. An unpermitted unit can cause problems when you sell, insure or rent. California’s AB 2533 created a path to legalize some older unpermitted units, but it does not cover new construction; see how to legalize an unpermitted ADU.
• Structural and foundation work. If an old garage needs retrofitting, skimping creates risk.
• Fire and life safety: smoke and carbon monoxide alarms, egress and required separation.
• Licensed trades for electrical, plumbing and gas.
• Insulation and energy compliance, which affect comfort and approval.

In contrast, finishes are the safest place to save: flooring, cabinets, fixtures and appliances can be upgraded later at far lower risk than structure or utilities. Simple rectangular footprints and standard window sizes also tend to cost less, though we have no sourced figure, so ask your builder.

Frequently Asked Questions

Which type is the cheapest way to build an ADU?

A junior ADU or a garage conversion, because they reuse existing structure.

Is a prefab ADU cheaper?

Not always. Instead, compare the total installed price, including foundation, crane and utility work.

Can I build an ADU myself to save money?

Owner-builder rules vary by jurisdiction, and permits and inspections still apply. However, we have not covered owner-builder projects in this guide.

Which single choice saves the most?

Choosing a conversion over a new detached unit.

Next Steps

• ADU Budget Breakdown: Three Sample Budgets
• ADU Design Cost: Architect vs. Pre-Approved Plans
• ADU Impact Fees: When You Pay Them and How to Avoid Them
• California ADU Laws 2026

General information, not financial advice. Costs vary by site; get written bids.

Sources: ADU Scale: California ADU cost guide (vendor, updated June 2026) · The ADU Pro: cost of hiring an architect · IMKAT Construction: ADU utility connections · Terner Center and CCI: First statewide ADU owner survey (2021) · HCD ADU Handbook, March 2026