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Accessory Dwelling Unit (ADU) Building Cost Guide

All costs updated as of 5/7/2021

Budget

The very first step in the process of designing your accessory dwelling unit is to establish the budget for the project. 

Once the budget has been established you need to determine what type of finishes you want in the ADU such as quartz countertops, stain grade doors, crown molding, tile flooring, etc. Once the level of finishes has been established you can figure a rough cost per square foot that it will take to build that type of ADU and then divide that out by your total budget number to see what size ADU you will be able to build with those finishes. 

During the design process, you will work directly with your design-build contractor who will make sure that your ADU is designed to meet your needs and budget. It is absolutely vital that you work with a contractor from the very beginning of the design phase in order to make sure the plans and finishes are designed to meet your budget. We have seen it happen many times where a complete set of plans have to be thrown away and a smaller or less elaborate accessory dwelling unit designed because the costs to build the original project came in much higher than expected.  

Plans & Design (Soft Costs)

The initial costs involved to get your accessory dwelling unit designed and the plans drawn will run between 4% to 6% of the overall cost to build your ADU. So if the total cost to build your ADU was $400,000 you could expect to spend $16,000 to $24,000 to get the design, budget, selections, plans, structural engineering & energy calculations completed & ready to submit to the local building department. 

The costs vary depending on if you hire a draftsman or architect and how complex the structural engineering is. 

If you choose to use one our pre-designed ADU plans and specification packages the plans & design will will run between 3% to 5% of the overall cost to build your ADU. This could save you $6,500 or more on this part of the project.

Plans & Design Example #1:

Here is an example of the design fees for a 1,200 sq. ft. ADU with a 576 sq. ft. garage & 200 sq. ft. of covered porch using one our pre-designed  ADU plans & specification packages:

$7,500     Contractor, Value, Engineering,  Design, Budgeting & Selections

$2,000     Plans (Pre-Designed Plans & Specs)

$750        Structural Engineering

$250        Title 24 Energy Calculations

$1,000     Interior Design

$11,500 – Total

Plans & Design Example #2:

Here is an example of the design fees for a 1200 sq. ft. ADU with a 576 sq. ft. garage & 200 sq. ft. of covered porch using a draftsman to draw the plans:

$10,500     Contractor, Value, Engineering, Design, Budgeting & Selections

$3,000     Plans (Draftsman)

$1,976     Structural  Engineering

$250        Title 24 Energy Calculations

$2,000     Interior Design

$17,726 – Total

Plans & Design Example #3:

Here is an example of the design fees for a 1,200 sq. ft. ADU with a 576 sq. ft. garage & 200 sq. ft. of covered porch using an architect to draw the plans:

$7,500     Contractor, Value, Engineering, Design, Budgeting & Selections

$6,000     Plans (Architect)

$1,976     Structural Engineering

$250        Title 24 Energy Calculations

$2,000     Interior Design

$17,726 – Total

ADU Permits & Fees (Soft Costs)

Permits and fees including school fees, traffic impact fees, building plan check fees, park fees, building permit fees, septic layout & septic permit fees will run between $9,000 to $12,000 depending on the size of the ADU and its location. 

School fees are charged on a per square foot basis and will vary for different school districts. If your ADU is less than 750 sq. ft. you may not have to pay school fees. 

Traffic impact fees also vary depending on what location you are in. The County of San Diego has currently waived all developmental impact fees until January 1st 2024. These include the park fees and traffic impact fees mentioned above. 

The numbers above include permits and fees for installing a private septic system & drilling a private water well. If you do not need a private septic system or water well installed you will need to contact your local sewer & water municipality to find out what the costs will be to tie into the city sewer system & water supply. 

Currently most sewer & water municipalities are not charging any fees for a homeowner to connect a newly built accessory dwelling unit to the current sewer connection and water meter that are servicing the existing primary dwelling. If you wish to separately meter the water for your ADU you may have to pay for another water meter if you are on city water. Installing another water meter can cost $18,000 or more.

ADU Permits & Fees Example #1:

Here is an example of the fees for a 1200 sq. ft. ADU with a 576 sq. ft. garage on a septic system that is plan-checked through the San Diego County building department and located in Ramona, CA.

$4,176    School Fees @ $3.48 sq. ft.
$0           Traffic Impact Fees (Waived by COSD)
$0            Building Plan Check Fees
$2,448     Building Permit Fees
$0            Park Fees (Waived by COSD)
$0            Septic Layout & Permit
$1,000    Permit Processing

$7,624 – Total

Site Costs for ADU's

The site costs for building an accessory dwelling unit will vary greatly depending on the land that you will be building on and how your current primary dwelling is situated on the lot. The site costs will include grading, soils testing, BMP’s (erosion control), sprinklers, landscaping, water storage tanks, booster pump and pressure tanks for water storage tank, septic system, electrical service line, solar system, propane or natural gas line, city sewer line connection, water meter connection, asphalt or concrete driveway and area drains. The average costs for all of these site costs typically fall between $30,000 to $80,000. 

If the primary dwelling on your property is currently connected to the city sewer you will not need to put in a new septic system or modify your existing one. This will save the $10,000 to $15,000 that it costs for septic, minus whatever costs are involved to trench & tie into the existing sewer line for the primary dwelling. The sewer district will most likely charge you for an additional EDU. This is very often attached to your property tax bill. You will want to check with them to see how much the additional EDU will cost you each year.

If your primary dwelling is on a septic system and you are on a smaller lot of an acre or less you may have difficulty fitting the required size septic system for the ADU on the property. One of the first things you need to do if this is the case is to contact the County of San Diego and check to see if they have a septic asbuilt on file for your property as well as the percolation data. This will allow the requirements to be evaluated and determine the feasibility of adding the ADU to the property.

If your primary dwelling was built prior to the County of San Diego requiring a 10k water storage tank for fire suppression and you do not have one or are not close to a city fire hydrant you will need to install one. This can cost anywhere between $10,000 to $15,000 including the tank, booster pump, hydrant & underground trenching.

As of January 1st 2020 a solar system now has to be installed on the property for any new accessory dwelling unit that is constructed. The exact size of the solar system is calculated in the Title 24 calculations and typically adds $10,000 to $15,000 to the total site costs. The solar panels can be ground mounted or mounted on the ADU.

Below are some rough costs on what you can expect to spend on site costs. Utility connections are not included as they vary greatly for each project.

 Site Costs Examples for an ADU

$4,000 to $7,000 – Over Excavation & Recompact (Grading)

$2,000 to $6,000 – Soils Testing

$1,000 to $8,000 – BMP’s (Soils Erosion)

$10,000 to $18,000 – 10K Water Storage Tanks, Booster Pump & Hydrant

$10,000 to $15,000 – Septic System

$10,000 to $15,000 – Solar System

$4.00 – Per sq. ft. – Asphalt Driveway

Build Cost for ADU's

When we are figuring the costs to build an accessory dwelling unit we will look at similar projects we have built with similar finishes and evaluate the cost of three different parts of the ADU. They are the living area, covered porch area and the garage size. The most accurate way to get a rough or ballpark number on what it is going to cost to build your ADU is to break the ADU into these three parts . The reason for doing this is you can have a 1200 sq. ft. ADU with a 1000 sq. ft. wrap around porch and a two car garage and it will cost more to build than the exact same ADU and garage with only 250 sq. ft. of covered porch. Similarly if you had a 1200 sq. ft. ADU with a 1000 sq. ft. garage it is going to cost more to build than the same ADU with a two car garage that is 576 sq. ft. Another example is if you have a 1200 sq. ft. ADU with no garage it will be much cheaper to build that ADU. So using a blanket price per square foot to estimate the costs to build a custom ADU is just not an accurate way to figure the costs. Again, by breaking the costs into the three different parts of living area, covered porch area and garage area you are able to get a much more accurate cost projection for building a custom ADU.

Price Per Square Feet Examples to Build an ADU

  • $250 to $350 sq.ft. – Living Area
  • $65 to $85 sq.ft. – Covered Porches
  • $90 to $125 – Garage (Attached) 

What affects price per square foot?

There are three other additional factors that can greatly affect the price per square foot to build an accessory dwelling unit.

1.The first factor is the actual square footage itself. If you build a 2 bedroom 2 bath ADU that is 1200 sq. ft. the cost per square foot to build that ADU will be cheaper than if you built a 2 bedroom 2 bath ADU that is 800 sq. ft. The reason for this is the smaller ADU has the exact same size kitchen and the exact same quantity of bathrooms. The kitchen and bathrooms are the most expensive areas of the ADU to build. When you take the same size kitchen and the same quantity and size of bathrooms and put them into the ADU with the smaller square footage the price per square foot will be higher on the smaller ADU. The cheapest square footage to build in the living area of the ADU is “open footage” which would be open area in the living room, hallways or bedrooms. These areas do not have cabinets, plumbing, countertops or electrical and are much cheaper to build per square foot.

2.The second factor that can greatly change the cost per square foot to build your accessory dwelling unit is the number of bathrooms you have. Since the bathrooms are some of the most expensive parts of the ADU to build, the price per square foot of the 1200 sq. ft. ADU would be lower if it had 1 bathroom versus the same size ADU with 2 bathrooms. Secondary bathrooms typically add $25,000 or more to the overall cost of an ADU. An additional master bath can add $30,000 on the low end and much more depending on how elaborate the design is.

3.The third factor that can greatly change the cost per square foot to build an accessory dwelling unit is the type of materials you choose to finish your ADU. These include exterior and interior finishes. For example: an ADU with a lot of stone veneer on the exterior, an elaborate siding design or Santa Barbara finish on the stucco will definitely increase the cost to build the ADU versus the same ADU without those finishes. Similarly on the interior of an ADU if you have wood windows, open beam ceilings, stain grade doors, stain grade wood trim, all tile floors, a smooth level 5 drywall finish, radiant floor heat and elaborate tile showers in all the bathrooms the price per square foot will be much higher than an ADU with fiberglass showers, LVP flooring, vinyl windows and drywall ceilings.

ADU Building Costs

Below are some rough cost examples of what you can expect to invest to build an accessory dwelling unit. These costs do not include the “plans & design”, “permits & fees” or “site costs” mentioned above. These numbers are strictly for the costs to build the structure of the ADU.

$357,820 – Price

2 – Bedrooms
2 – Bathrooms
1095 sq. ft. – Living Area
462 sq. ft. – Garage
197 sq. ft. – Covered Porch
Average – Level of Finishes

$335,100 – Price

2 – Bedrooms
1 – Bathrooms
1087 sq. ft. – Living Area
337 sq. ft. – Garage
64 sq. ft. – Covered Porch
Average – Level of Finishes

Price- $307,040

1 – Bedrooms
1 – Bathrooms
763 sq. ft. – Living Area
477 sq. ft. – Garage
505 sq. ft. – Covered Porch
Average – Level of Finishes

Price- $398,290

2 – Bedrooms
2 – Bathrooms
1200 sq. ft. – Living Area
478 sq. ft. – Garage
352 sq. ft. – Covered Porch
Average – Level of Finishes

Price- $322,455

4 – Bedrooms
2 – Bathrooms
1113 sq. ft. – Living Area
252 sq. ft. – Covered Porch
Average – Level of Finishes

Price- $337,800

3 – Bedrooms
2 – Bathrooms
1200 sq. ft. – Living Area
120 sq. ft. – Covered Porch
Average – Level of Finishes

Price- $432,230

2 – Bedrooms
2 – Bathrooms
1188 sq. ft. – Living Area
506 sq. ft. – Garage
884 sq. ft. – Covered Porch
Average – Level of Finishes

Price- $357,760

2 – Bedrooms
2 – Bathrooms
1157 sq. ft. – Living Area
164 sq. ft. – Covered Porch
Custom – Level of Finishes

Price- $383,400

3 – Bedrooms
2 – Bathrooms
1200 sq. ft. – Living Area
480 sq. ft. – Garage
120 sq. ft. – Covered Porch
Average – Level of Finishes

So to recap how to get the total costs to build your accessory dwelling unit (ADU) you will need to add up the following:

4% TO 6% – PLANS & DESIGN

+ PERMITS & FEES
+ SITE COSTS
+ BUILD COSTS

    =

TOTAL COST

TO BUILD

YOUR ADU

You will also need to factor in the cost to obtain financing if you are going to need a loan to complete the project.

Freeman’s Construction Inc. has been building custom homes, accessory dwelling units, and completing whole house remodeling projects in San Diego County for over 39 years. To schedule a project consultation please call us today at 760-788-6846 or fill out a request form here.

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