How to build an Accessory Dwelling Unit (ADU). A step-by-step guide to planning and building an ADU in Los Angeles, California.

Accessory Dwelling Unit (ADU) Developments Can Be Very Profitable!

Taking a piece of real estate and improving it by adding an ADU is a relatively easy form of Real Estate Development that even a novice real estate investor or homeowner can accomplish with the right guidance.

Over the course of the next 12-16 weeks I will take you along as I build a brand new 1,200 sqft ADU on a property in Hyde Park, Los Angeles. I will be sharing a step-by-step "how to" which covers the entire ADU construction process from the initial design to final inspection.

Farmhouse ADU Being Developed in Los Angeles, California

This ADU is being constructed on a property I own, and I am acting as my own General Contractor. Although I am a licensed General Contractor, the information I provide in this guide will benefit anyone who is ready to start building their own ADU, even if you have little or no experience in construction. If you have a willingness to learn and are not afraid of a little hard work, then you can build your own ADU which you can use for investment or personal enjoyment. Or both.

If you have any questions or would like to discuss your own ADU construction project, please do not hesitate to contact me with questions or comments. I look forward to sharing this process and I encourage all homeowners and investor who can add an ADU to their property to do so. Southern California needs more housing and we can help by providing additional housing options for our fellow Angelenos.

Step #1 - Choosing the type of ADU that is best for you. Attached vs Detached vs Garage Conversion.

The ADU I am building will be used as a rental, so I wanted to maximize my return on investment. With that as my primary motivation I chose to design and build the largest ADU that was allowed under the City of Los Angeles' ADU Ordinance.

Larger ADUs tend to rent for more money, and every square foot of new construction you add to your property translates into a higher resale value once you decide to sell.

I wanted a 3bd/2ba ADU that I could rent for top dollar. I chose to build a detached unit because I had room in my backyard, and I thought that a detached ADU would be more desirable for a family looking for a home. And as any real estate investors knows, families tend to stay longer which equals less tenant turnover which equals less headaches and lost revenue due to tenant turnover.

Once I settled on the type and size of ADU I wanted to build the next step was to get a design in place and ADU plans and construction drawings produced.

I knew exactly what I wanted and since the ADU was only two stories I didn't need a licensed Architect to come up with a new ADU design, I simply hired an experience draftsman to prepare the plans, and Structural Engineer to provide structural calculations which are typically required.

It is particularly important that you hire a draftsman or architect that is experienced in designing ADUs, and experienced processing ADU plans in the city you live in.

DO NOT try to save a few dollars by hiring someone that does not have experience with ADUs, or an Architect or designer that is not familiar with the ADU ordinance in your city. An inexperienced designer, Architect, or Engineer can add months to your project timeline if they are not familiar with the ADU ordinance and how each city treats the permitting process.

Once the Architect is done with the ADU plans the next step is to submit the plans to the city building department and wait for their comments or approval. In my case it took the City of Los Angeles about 30 days to review my plans, and after I made a few minor corrections to the plans based on the cities feedback, I had permits in hand and was ready to start construction.

Step #2 - Building Your New Accessory Dwelling Unit

  • Site Clearing & Building Pad Preparation

    The main house was built it 1914 and was an old craftsman style bungalow. At the time it was built it apparently had a two-car detached garage space, but at some point over the last 100 years the garage had been demolished or destroyed. I am not sure what happened to the original garage, but it wasn't standing when I bought the property in the Summer of 2019. The only evidence that a garage once existed was contained in old building permits from 1914 that showed the size and location of the original construction. It was the existence of these old building permits that lead to the first delay in the construction of my new ADU. Since the building departments records showed the existence of a garage on the property, I was required to pull a demo permit to remove the garage. A garage that no longer existed on the property. And to top it all off, the City of Los Angeles required that I send all my neighbors within 500 feet a notice notifying them that I was going to demolish the garage and give them 30 days to voice any concerns regarding the demo to the city. Wait, what? You want me to pay for my demo permit, send my neighbors a "notice of pending demolition" and waited for 30 days to start my project so that my neighbors can protest the demolition of my garage that doesn't exist? Yes, that is what they required. A complete waste of time and money since I had to pay 30 days worth of mortgage expenses and taxes waiting for a demo permit for a garage that had long ago been demolished. Oh well. Unfortunately these types of delays are inherent in the construction process and is why a contingency of 10-15% of your construction budget is so important.

    I finally had the permits complete and was ready to start building my ADU. The backyard is almost completely flat, so my engineer designed a slab-on-grade foundation for the ADU. This is the least expensive foundation to build and is perfect for an ADU foundation on a flat property. But slab-on-grade foundations require a flat surface to build on and so I went to work clearing out all the trees and vegetation that was in the footprint of my new ADU. You will see from the picture below that there was one old tree that was the biggest obstacle. A quick call to a tree trimmer I found on Craigslist and the tree was removed and hauled away $1,100. That was a bargain and much less expensive than the $3,500 I had budgeted for this line item..

    Removing a large tree in the footprint of the ADU foundation.

  • Foundation Footings

    The engineer designed a 6" thick slab with 18" wide and 18" deep footings. Since the dirt was relatively soft and the foundation area relatively small, I proceeded to hand dig the footings as the cost of renting equipment would have doubled the overall cost of digging the footings. I used two hard working construction laborers to help, and with my guidance I was able to dig the entire foundation and prepare the footings in a little under 16 hours of work. Now that we have the footings dug it is time for the next step… underground utilities.

  • Underground Utilities

    At this point in the build it is necessary to run any underground utilities that will run underneath the foundation slab. For this build I have chosen to run the property entirely on electric power. I chose to use electric only to save a little money on the build. Running only electric to the ADU meant I did not need to run gas lines which saved about $3,500. This decision means that the only underground utilities I have running under the foundation slab and through the footings was my plumbing waste lines.

    The picture below shows the underground plumbing lines installed under the slab. At this point we called the city inspector to come out and approve the foundation pad and footings, and to perform a water test and approve the underground plumbing. With the inspectors blessing we back filled the sewer trenches and started progress on preparing the slab for our concrete.

    Foundation Footings and Forms For Our New ADU in Los Angeles

  • Adding Structural Rebar

    After the underground plumbing was complete and the plumbing lines backfilled with soil and compacted, it is now time to place the rebar to our footings as designed by our Structural Engineer. The rebar is structural steel that is placed inside the concrete footings and provides the concrete with added strength. In addition to rebar in the footings, the engineer also called for the slab to be reinforced with wire mesh, as well as a vapor barrier over the soil which would prevent moisture from the ground soaking into the slab from the bottom.

    The picture below shows what the foundation looks like just before we placed the concrete.

    Slab on grade preparations for an ADU in Los Angeles
  • Placing the Concrete

    After we placed the rebar into our footings, the with vapor barrier and wire mesh across the slab area it was time to pour our concrete. We called the city inspector out to the job site to approve the rebar, vapor barrier, and wire mesh. The inspector gave us his blessing and told us to call him back out after we were done with the rough framing portion of the project.

    A quick note on inspections... never cover up any work until that work is seen and approved by a city inspector. So, in this case when we pour our concrete we are going to cover up the rebar, vapor barrier, and wire mesh work we performed, so it was necessary to call out the inspector before that work was covered. Once the concrete is placed into the ground the work gets covered up and there is no way for the inspector to verify we performed the work to code or the approved structural design. Always, always, always call the inspector before you cover up you work. You will see as we progress on the project that the inspector is always called out right before we cover work.

    Back to pouring the concrete. The engineer called for a standard 2,500 PSI concrete and our calculations showed that we needed about 27 yards of concrete to complete the slab. We hired a skilled set of concrete finishers to place and finish the concrete. A crew of four men were able to get the concrete poured and finished in about 4 hours.

    The picture below shows a progress picture of the concrete being placed into the footings and slab.

    Slab on grade in progress for an ADU in Los Angeles
  • Stripping the Forms and Framing Layout

    I let the concrete set-up and dry over the weekend and came back of Monday to strip the wood forms from the foundation and to perform framing layout with my framing crew who would be performing the rough framing for my ADU.

    You will see in the photo below that there are bolts sticking up from the concrete slab at the perimeters of the foundation. Those bolts (called "hold downs") were installed prior to the slab being poured and will be hold the foundation to the rough framing. This is a crucial step for the structural integrity of the new home and is required wherever you build, but these bolts are especially critical in California where earthquakes can easily shake a home of its foundation.

    Slab on grade in progress for an ADU in Los Angeles
  • Rough Framing

    The next step in the ADU construction process is the Rough Framing. This is when the construction of your ADU gets fun as you see the new home rise out of the ground. Us builders refer to this stage as "going vertical".

    I specifically designed this ADU to be inexpensive to build by using off-the-shelf products and traditional dimensional lumber with no Structural Steel framing or large structural beams. As such my framing crew will be able to complete the rough framing portion of this project in about 12-14 working days. My job now is to make sure the framers have the lumber and materials they need, and to make sure my windows are ordered as we'll need the windows after the rough framing is complete.

    The video below shows the progress after 5 working days. It's amazing how fast a small home like this can be built with an experienced crew of framing carpenters.


    And these pictures show the progress after 10 working days. At this point the crew needs to finish the roof rafters, roof sheathing, sheer walls, and perform the framing pick-up. Three more full days and the framers will be 100% complete with the rough framing portion of the project.

    And here are a few pictures from four days later when the rough framing for our ADU is 100% complete.

    Insider Tip : Always ask the rough framing crew to install your vinyl windows as part of their service. Most rough framing carpenters are experienced installing standard vinyl windows and will include this service for a nominal upcharge. It will be much less expensive then hiring a dedicated window installation subcontractor to come in after the framers are finished to install the windows. But this advice only applies to standard vinyl and builder-grade windows. Custom windows should be left to professional window installers.

    With the rough framing now complete it is time to get the plumber, electricians, and HVAC subs started with the installation of their rough work. These trades are referred to broadly as the mechanical trades.

    At this stage in the project the mechanical subs are running all their work that will be within the interior and exterior walls of our new ADU. All the water supply line, sewer drain lines, electrical wire, and HVAC supply and return lines are being installed now.

    We scheduled two full weeks to let the mechanical subs complete their rough installation at which point we will need to call the city inspector back out to inspect and approve their work, at which point we'll be ready to insulate the attic and walls and begin the drywall

  • MEPs

    At this stage in the project the MEP subs started. What's an MEP? Good question. MEP stands for Mechanical, Electrical, and Plumbing. These subs are lumped together because their work will is all happening at basically the same time.

    The MEP subs are running all their work that will be within the interior and exterior walls of our new ADU. All the water supply line, sewer drain lines, electrical wire, and HVAC supply and return lines are being installed at this point in the project.

    Your building plans should contain all the MEP specifications so that the subs know where to install their work. Without good plans the subs will be bumping into each other all over the place. For example a plumber might run a waste line where an electrician is supposed to install a recessed light fixture. These issues can be kept to a minimum with a good MEP plans.

    We scheduled two full weeks to let the MEP subs complete their rough installation at which point we will need to call the city inspector back out to inspect and approve their work.

  • Siding

    For the siding I chose to use a board and batten style cement siding by James Hardie. It will give our ADU the very popular Modern Farmhouse look, and will be a very long lasting option.

Our next update...

Next week we will have pictures of the completed rough mechanical systems (i.e. - Electrical, Plumbing, and HVAC). We'll see you in a week or so with an update.

If you're thinking of building an ADU anywhere in the Los Angeles contact us. We're helping homeowners just like you all over Los Angeles build ADUs and Granny Flats.