How Much Do Adu Cost? | Real Numbers By Build Type

An accessory dwelling unit (ADU) can be a garage studio, a backyard cottage, or a small wing attached to the main house. If you keep asking yourself “how much do adu cost?”, you are not alone. People search this question because budgets swing fast, and a clean number on a screen rarely matches a real bid. This page gives you ranges you can price-check, plus the line items that usually move a quote up or down.

Use these ranges to plan, then confirm with bids.

How Much Do Adu Cost? Cost Ranges At A Glance

Start with the build type, then adjust for site work, utility runs, and finish level. The table below is meant for early budgeting, before you have drawings and contractor bids.

ADU type	Common size	All-in cost range
Garage conversion (basic)	300–500 sq ft	$80,000–$160,000
Garage conversion (with new bath + kitchen)	400–700 sq ft	$120,000–$220,000
Basement conversion	500–900 sq ft	$140,000–$260,000
Attached addition (one-story)	500–900 sq ft	$170,000–$320,000
Detached new build	500–1,000 sq ft	$200,000–$450,000
Detached two-story	700–1,200 sq ft	$280,000–$600,000
Prefab/modular set on a foundation	400–1,000 sq ft	$160,000–$380,000
Junior ADU (inside primary home)	150–500 sq ft	$40,000–$120,000

These ranges bundle construction, a standard finish package, and common soft costs. They assume you already own the lot. Land purchase is its own math.

What Most Quotes Include And What They Skip

Two estimates can look close while covering different scopes. When you compare bids, check whether each one includes the items below in writing.

• Hard construction: framing, roof, windows, insulation, drywall, flooring, paint, cabinets, fixtures, appliances, and HVAC.
• Site work: demolition, grading, drainage, retaining, paving, and fencing repairs.
• Utilities: trenching, new water and sewer lines, electrical upgrades, gas, and a new panel or subpanel.
• Soft costs: design, engineering, surveys, energy forms, permit fees, school or impact fees where charged, and city inspections.
• Project overhead: contractor supervision, scheduling, and general conditions like temporary power and toilets.

Common “gotchas” that show up after you sign: a tighter fire separation than expected, a utility service upgrade, soil fixes, or a driveway that must stay code-compliant after trenching.

Price Drivers That Move An ADU Budget Fast

Most of your cost swing comes from a handful of choices and site facts. If you want a fast sanity check on a contractor’s number, work through these levers.

Size And Layout Efficiency

Cost per square foot drops as the unit gets larger, but total cost rises. A compact plan that keeps plumbing close together often beats a larger plan with long pipe runs and extra corners.

Plumbing And Utility Distance

The farther your ADU is from the main sewer, water, and electrical service, the more you pay in trenching and restoration. A detached backyard unit can be cheap to frame and still end up pricey once you add a long utility run and concrete replacement.

Foundation And Soil Conditions

Sloped yards, expansive clay, and poor drainage push you toward deeper footings, more rebar, and extra drainage work. If your area commonly needs a geotechnical report, budget for it early so it does not derail permitting later.

Finish Level

Finishes are where “small” changes stack up: tile that needs extra prep, custom cabinets, stone counters, upgraded windows, and higher-price finishes. A simple rule: pick one splurge category, keep the rest mid-range.

Local Fees And Review Steps

Permit fees vary by city and county. Review time can also carry a cost because longer schedules mean more months of rent elsewhere, storage, or paying interest on borrowed funds. In California, the state ADU materials from the Accessory Dwelling Unit Handbook can help you spot common permit steps and terminology.

Cost Breakdown: A Realistic Line-Item Budget

If you want a working budget, split your estimate into buckets. You can then adjust each bucket as you make choices.

Construction Core

For many projects, the core build lands in a wide band of about $150 to $300 per square foot. High-cost markets, tight sites, or higher-price finishes can push past that. Conversions can land lower because you reuse walls, a slab, and sometimes the roof structure.

Site Prep And Access

Contractors price access because it changes labor hours. A narrow side yard that blocks a mini-excavator means more hand work. Street parking limits can add daily shuffling, which shows up in general conditions.

Utility Upgrades

Many homeowners get surprised by the main electrical panel. If your house has a 100-amp service and you add an electric water heater, mini-split, range, and dryer, you may need a service upgrade. Sewer laterals can also be old or undersized. When upgrades are needed, costs jump quickly because the scope involves coordination with the utility provider and extra inspections.

Design, Engineering, And Permits

Plans, structural work, and energy documents are not optional. Some owners try to trim this area and pay later through redesigns. A clean plan set that answers plan-check questions on the first pass often saves weeks.

Contingency

Set aside money for unknowns. On conversions, surprises often hide in framing, old wiring, and moisture. On new builds, surprises tend to be soil, drainage, and utility depth.

Ways To Lower Cost Without A Cheap Feel

You do not have to slash quality to protect your budget. The moves below reduce labor hours, reduce rework, or avoid expensive change orders.

Keep The Footprint Simple

Rectangles are friendly to framing, roofing, and waterproofing. Extra jogs and bump-outs add corners, flashing, and time. If you want outdoor space, consider a small patio instead of a complex shape.

Stack Wet Areas

Place the kitchen back-to-back with the bathroom wall when you can. Shorter runs mean fewer vents, fewer penetrations, and easier inspection.

Pick Stock Sizes

Standard window sizes, standard door widths, and a cabinet layout that fits off-the-shelf boxes keep materials lead times sane. Custom work can look sharp, but it can also drag a schedule out when one item ships late.

Decide Early

Change orders are where budgets get messy. Create a finish schedule before framing starts: flooring, tile, paint sheen, cabinet color, fixtures, and appliances. Then stick to it.

Financing And Payback: What To Run Before You Build

Cost only matters next to your plan for the space. A unit for family may justify a higher finish level than a rental. A rental plan needs a sober rent estimate and a plan for vacancies.

Common Funding Paths

• Cash savings: simplest paperwork, no interest cost, slower to accumulate.
• Home equity loan or HELOC: flexible draws, rates vary, payment starts fast.
• Cash-out refinance: may work when rates suit you, closing costs apply.
• Renovation loan programs: lender rules are detailed, appraisal matters.

If you are shopping mortgage options tied to an ADU, Fannie Mae’s own overview on Accessory Dwelling Units (ADUs) is a solid starting point for lender terminology.

Simple Payback Math

Run three numbers: your all-in cost, your realistic monthly rent after vacancy, and your monthly loan payment. If the rent only covers part of the payment, decide if the rest still fits your household budget with room for repairs.

Bid Types That Change The Final Number

You may see three pricing styles. A general contractor bid is a fixed scope and price. A design-build team bundles drawings and construction, which can cut redraws. A modular quote may cover the unit only, then you pay for foundation, hookups, and site repairs. Ask what is included, what is an allowance, and what triggers added charges.

Timeline Costs You Should Not Ignore

Time is money in construction. Even when the contract price is fixed, a longer schedule can add real costs you feel day to day.

• Permitting time: extra months can mean more rent, storage, or living disruptions.
• Material lead time: special-order windows, custom cabinets, and niche tile can stall work.
• Weather delays: rain slows concrete and framing, especially on sites without covered storage.
• Inspection gaps: missed inspections can force rework or idle crews.

A good builder gives you a schedule with milestone dates and a clear list of owner decisions needed to stay on track.

Cost Checklist For Comparing Contractor Bids

Use this checklist to compare bids line by line. It also helps you see which items you still need priced before you feel confident signing.

Budget area	What it covers	Typical range
Design + engineering	plans, structural, energy forms	$6,000–$20,000
Permits + plan check	city fees, reviews, inspections	$3,000–$15,000
Site work	demo, grading, drainage, access	$5,000–$40,000
Utilities	trenching, tie-ins, panel work	$8,000–$45,000
Construction shell	foundation, framing, roof, windows	$60,000–$220,000
Interior finish	drywall, cabinets, flooring, paint	$35,000–$160,000
Appliances + fixtures	range, fridge, bath, lighting	$5,000–$35,000
Contingency	unknowns and scope creep buffer	8%–15% of total

Quick Steps To Get A Tight Quote

If you want numbers you can trust, treat your first quote like a draft, not a verdict. These steps tend to tighten pricing fast.

1. Pick the ADU type: conversion, attached, detached, or modular.
2. Measure the utility distances and note any concrete that would need replacement.
3. Set a finish target with photos of styles you like, then cap your must-haves.
4. Ask each contractor to list exclusions in plain language.
5. Keep your scope stable until after foundations and rough-ins are complete.

When friends ask, “how much do adu cost?”, the honest answer is “it depends,” but you can still budget with confidence. Start with the build-type range, price utilities and site work early, then keep your finish choices consistent. That’s how you land a number that matches the check you will write.