ADU Cost: how much does it cost to build ADUs in California?

One question an ADU expert is most terrified of.

Typical ADU Cost .

How much does an ADU cost per square foot?

Homeowners always ask this question.

The quick answer is I do not know. There are so many variables in every case, there is no typical ADU cost.

The bad answer is around $500 per square foot on average, often more for modular units.

The good answer is more elaborate. The budget for this prefab ADU (see the photo for the two bedroom one bath unit) on a flat lot with a deck and a panel upgrade was $230,000 for 450 sq ft in 2021. Which means $513 per sq ft. The average price per sq ft of living space in that neighborhood per Zillow is $770. Rents for $2,500 per month.

Annual taxes and maintenance expenses excluding, this ADU may return the capital investment in 8 years.

That said, in 2023 all-in pricing for the same unit starts at $280,000.

How much does a modular  ADU cost per square foot?

A quick surf on an Abodu website shows $628 per square foot for their 500 sq ft unit. Samara‘s 540 sq ft unit starts at $550 per sq ft.

Small 570 sf ADU
Modular ADU completed

How much does a manufactured  ADU cost per square foot?

Villa Homes sells their 650 sq ft unit at $420 per square foot.

Mind all these numbers show only starting prices (checked on September 23, 2023) – costs may be higher for your property. It all depends on access, soils, slope, length of trenching for utility lines, their condition and other items that may impact your ADU cost.

If you are interested in modular construction, make sure to visit several showrooms before you make a decision.

How much does a custom  ADU cost per square foot?

It depends. Starting from $425 per square foot, and the sky is the limit.


The real answer for your ADU on your property depends on the answers to a dozen of questions.

Question #1

How big is it going to be?

ADU Floor Plans 500 sq ft

The first 300 square feet are the most expensive in any building. Costs per square foot will be higher in smaller homes. Utility trenching is the same whether you build a small home or a big one.

800 sq ft ADU Floor Plan

This two bedroom one bathroom ADU will give you a better cost per square foot. Notice that most plumbing is concentrated in one corner of the house.

1200 sq ft ADU Floor Plan

This two bedroom two bathroom ADU has a flex room for a home office. It will give you the best value for money – as long as you have that much space in the backyard.

500 sq ft ADU

Starting at

  • One bedroom, one bathroom
  • One French Door
  • Heat pump water heater and HVAC

800 sq ft ADU

Starting at

  • Two bedrooms, one bathroom
  • Laundry
  • Heat pump water heater and HVAC

1200 sq ft ADU

Starting at

  • Two bedrooms, two bathrooms
  • Laundry, home office 
  • Heat pump water heater and HVAC

Reality Check

ADU Cost Breakdown

There are multiple line items that make the total ADU cost. It starts with pre-construction costs. Those include design costs, engineering costs, permit costs. Construction costs have a longer list – from foundation all the way to cabinet and appliances installation.

Our proposals typically contain about 30 line items. If you are not on a spending spree with your ADU, it is important to make sure every line item makes sense. 

ADU Design Cost

Sometimes your property shape drives custom design. In this case it may be worth to pay $20,000 or more for the plans prepared by the licensed architect. More often than not, you can get away with one of pre-designed plans, created by architects for other clients and tested in the field. 

ADU Architect cost

If you have set your heart on working with an architect, here are the architects who have really embraced the ADU movement.  Check the works of Inspired ADUs, Jennifer  Kretschmer, Jessica Resmini and Todd Jersey.

If you are on a budget or at least looking to get value for money, let us check if you need a licensed architect for your ADU. You may also move forward with an ADU design team – an architectural designer and a structural engineer.

Who Can Design Your Attached or Detached ADU?
calculating ADU cost with paper, pencil and calculator

What should an ADU permit set include?

Surprisingly, the same information that any new construction plan set requires.

Title Sheet

ADU floor plan

Roof Plan

Foundation Plan


Building Sections

Structural information

Energy efficiency report

Cost for ADU Plans

ADU blueprint plans are priced separately by the architects and designers, however they are often included in the cost of a prefab ADU.

ADU Permit Cost

Permit costs are issued by the local jurisdiction – your City’s or County’s Building Department. You are charged ~$1,500 at the start of the process for plan check. At the time of approval the jurisdiction will issue the remaining plan check and permit fees plus inspection fees, street impact fees, record management fees, and even a Tech Enhancement fee if you are in Oakland. Some fees are waived if your building is less than 500 or 750 sq ft.

ADU Fee Structure

Here are the items you may expect to see when you get the invoice from your city or county: application fee, inspection fee, plan check fee, address fee. Some cities may charge bedroom tax and tech enhancement fee (hello, Oakland). Smaller ADUs won’t be charged school fees. Utilities may charge additional fees for new connections.

Cost for ADU Permit

Here are the recent examples from three Bay Area cities for ADUs under 350 sq ft. Los Altos: $7,900. Menlo Park: $6,700. Oakland: $8,400.

ADU Cost to Build

Cost of adding ADU differs between SoCal and NorCal. Actually, ADU cost differs in Bay Area too.  One of our least favorite Pandora boxes is the charges from a utility company for connecting the ADU to water and sewer. A Los Gatos client has recently been charged ~$8,000. EBMUD invoiced an Oakland client  $17,000. 

ADU Foundation Cost

After receiving the plans from the architect or designer, your structural engineer runs structural calculations and designs foundation. Depending on foundation of existing house and soil conditions, your foundation can be a traditional slab, stem foundation or piers and beam foundation. 

ADU Framing Cost

Framing cost includes lumber material and labor. Your structural engineer will also call out shear walls and fasteners. If you are in a fire hazard zone or you were required to have a fire rated wall, you will be advised to use flame resistant material instead of traditional  OSB or plywood.

ADU Material Cost

After the framing is done, it is time for an insulation and drywall crew. Insulation value has been identified in your energy efficiency report. The architect may have called for regular batt insulation or a more expensive spray insulation. This is based on a trade off between size of studs and rafters and R value of the batt. It often makes sense to pay more for insulation to leave more space inside the house.

ADU Electrical Cost

You will be surprised how many electricians calculate their proposal based on number of outlets, switches and lights. You will be even more surprised, that these costs are often less than the cost of the underground wire that leads from the main panel to the new backyard home in the back. There are several items that can impact the electrical costs: timely upgrade of the main panel (200 Amps min) and informing your electrician on the model of appliances to be installed. This may have a huge impact on both time and budget.

ADU Plumbing Cost

Is there anything less sexy than talking about sewer lines when you meet people for the first time? Yet, this is something any ADU expert has to do as this is the biggest source of surprises. Will there be enough slope to avoid installation of a pump? Is the existing sewer line to main 4 inch to accommodate additional usage? Or is the property on septic? And that is a completely different story that can add thousands of dollars to your budget.

ADU Kitchen Cost

Finally, the cost a homeowner can control – IKEA or custom made cabinets? Base cabinets only with shelving or base and top cabinets? Butcher’s block countertop or a quartz one?

Hack your own house - Build wealth with the property you already own, perfectly suited to your needs.

Question #2

How is it going to be built?

ADU prefab wood panels
Prefab ADU Cost

Wood panels

The package for a prefab Adu will cost you more than a package of uncut lumber, but the crew will spend less time in your backyard. Every wall consists of several smaller panels that come preassembled and get erected in record time.

adu prefab with MnmMod
Prefab ADU Cost

Steel Frames

These function same way as wood panels, but are heavier plus fire & termite resistant. Account for longer lead times, as to keep costs down these will be shipped out from outside of Bay Area.

framing 1 bed ADU
Traditional Construction

Site Built Homes

Site built homes have one serious drawback – the construction happens in your backyard. Benefits include custom shape and control over the end product.

ADU exterior view (Connect home)
Modular Construction

Built in modular or manufactured home factories, these come in rectangular forms mainly, with fixed floor plans, with minimal customizations available.

Question #3

How many rooms and bathrooms?

ADU studio

Minimal size of an accessory dwelling unit by law is 150 sf ft. We are able to fit a bathroom and small kitchenette into a 200 sf ft studio. Expect to pay more per one square foot for a smaller ADU.

contemporary backyard house

The square footage of a 2 bedroom unit will start at 400 sq ft – and that will give you a tiniest living space, as we need to leave at least 70 square feet for each bedroom. ADUs can be as big as 1200 square feet. How big of an ADU can fit in your backyard? (See question #1)

studio ADU idea with bathroom and closet

Junior accessory dwelling units are converted from existing living space, so most of the time they are the most cost effective and fastest. Junior ADUs can use facilities of the main house thus eliminating the biggest budget surprises.

studio ADU space with yellow wall accents

While ADU studios often run small, the costs vary depending on types of doors and windows, storage (built-ins or lofts), utilities.

2 story home

A two story ADU is possible if your main house has two stories. In this case, it is often a space efficient solution that leaves more of a backyard for kids and dogs to roam.

The costs to convert a garage differ. The main trigger is the structural safety of the framing and foundation. See below for more details.

Before demo, consider the value of a garage in your neighborhood.

There are three possible scenarios with a garage conversion:


Scenario #1

  • Structurally safe building
  • Garage has windows
  • Existing water and sewer line 

Major Remodel

Scenario #2

  • Structure needs reinforcement
  • New egress windows 
  • Repairs to exterior 

Build New

Scenario #3

  • Tear down and rebuild
  • New foundation
  • New building envelope 

ADU Floor PLan 500 sq ft is a sweet spot

Is ADU a good investment?

Legally permitted ADU, built well and on a reasonable budget, is a good investment.

Legally permitted means you have applied for a building permit and, after the construction,  had it finaled by the building inspector. Built well – means built with materials of good quality and by an experienced crew. Reasonable budget means that whatever you have spent on the ADU has a chance to get reflected in the property value.

Three homes in place of one

The legislation passed by California in January 2020 made it possible to add more livable space to a current single family home. You might be surprised by the number of options this law has opened for an average homeowner, who has become a real estate developer overnight.

These options include newly constructed detached or attached ADU and conversion of existing space (garage or basement). You are allowed to build up to 1200sf. This is your living space #2. As an additional option, you may convert existing square feet into a Junior Accessory Dwelling Unit. This makes it your living space #3. Spread your family out or rent out.

ADU Floor Plans 500 sq ft

Challenges with financing

The challenge with some new financing solutions  is that the homeowner misses out on the lion share of the income and/or property appreciation. Your financing partner might be offering you  $300/mo income on the ADU, while if you were able to finance it yourself, your net income could be $850/mo. 

ADU size matters

Accessory Dwelling Unit is a small house. It needs utilities and foundation as much as your main house, but has less footprint to spread the costs over. Hence we always encourage you to build max possible.

Trenching for utilties costs the same whether it is for a 200sq ft ADU, 500sq ft ADU or 1200 sq ft ADU. This is the type of fixed cost that makes per square foot costs ridiculously high for smaller ADUs.

ADU project sounds like a challenge?

If you are in the Bay Area, we make it easy to get started.


Create property site plan

Let us answer three questions together: 1. How big can you build? 2. How much of a backyard you can dedicate to new living space? 3. How much you can afford to spend on construction.


Select the best ADU floor plan

We do not like to invent wheels or bicycles. Let us get started with the plan that fits most of your requirements and then tweak it to make it shine in your backyard.


Assign the team

The success of a construction project hinges on how well the team communicates. You have a contractor you like? Bring her on board. We are happy to support you with the design, engineering and construction – when needed.


Gather Necessary Documents

Let us start with the best information available. What do you already have? Title report. Floor plan or site plan of your main house. Survey. Soil report. All these documents are a great foundation for your future successful construction project. Pun intended.


Compile the permit plan set

The number of pages in your plan set can differ from 8 to 140 (no kidding). It all depends on property specifics and city (or county) requirements. The better prepared your first submission is, the faster you get your permits. 


Respond to Plan Checker comments

Do you know that the most frequent reason it takes such a long time to pull permits is the number of weeks that passed between the day the city had given plan check comments and the day your team responded to them?

image of a small ADU model and calculator