Did you know a garage conversion is the most cost-effective way to add an ADU to a property? While you may have to sacrifice additional parking and storage, the benefits of creating a livable space are hard to ignore.

And in some states, an ADU garage conversion is relatively easy. In the midst of a serious affordable housing crisis, California, for example, allows any property in a single-family residential zone to build a garage ADU. And many other state leaders have sought ways to create more homes and have agreed upon ADUs as one solution.

If you’re considering an ADU garage conversion on your property or need inspiration for garage ADU designs, here’s everything you need to know.

Do I need permission to convert my garage?

Short answer: yes, you need to make sure that local authorities in your area allow garage conversion ADUs.

In addition, depending on your neighborhood, you may also have to consider land boundary lines between you and surrounding homes and receive approval from your homeowners’ association members. Every city and county will have slightly different rules, ordinances, and regulations in regard to home renovations, so be sure to get local approval before you begin any construction.

Legalizing your garage

In most cases, the city and county where you reside will typically request copies of the original building permits to make sure that the existing structure (your home) is legal. This also means your garage must be brought up to code or returned to its original state when it was permitted before you can begin any work on a garage ADU.

For example, if you have a window that was placed out of code, you’ll either need to replace it or remove the window to return the wall to its original state. With any updates you do have to make, try to incorporate them into the garage ADU designs you have planned to maximize your investment in the renovation.

Submitting plans and getting inspections

When you’ve narrowed down your garage conversion ideas and are ready to move forward, you’ll need to obtain architecture and engineering plans that comply with local building codes. We recommend working with a professional who can help you submit the permitting plans and walk you through the inspection process. But here is what you can expect a typical inspection to include:

  • Existing structural elements that will remain in place during the garage conversion (e.g., the walls, roofs, etc.)
  • Foundations under the home
  • Structural tie-downs that hold walls to the concrete foundations
  • Wall and ceiling insulation
  • Any electrical and plumbing installations in the ADU, which includes solar and any off-grid installations

Once your local authority reviews and approves all plans and inspection information, they will issue a certificate of occupancy that is necessary in order for tenants or family members to move in.

Does converting a garage to an ADU add value?

First things first — to ensure the ADU garage conversion will be worth it, you’ll want to take a close look at the condition of the existing garage. If the framing, foundation, siding, or roof has degraded or is not up to code for a habitable dwelling, you will need to repair or replace them before you do anything else. And in these cases, you might be better off starting from scratch on another area of your property in order to reap the highest return on investment (ROI) when all is said and done.

However, if your garage is in good condition, converting this space to an ADU is one of the quickest and easiest ways to get a great ROI on your property. A lot of this has to do with the garage already existing, so a conversion saves money on the foundation, framing, and roofing portions that a typical construction project requires. You may also already have water and power lines running to the ADU — or even a sink or overhead lights — all of which save you costly steps in a standard renovation project.

Moreover, a garage conversion is typically quicker to permit, and the construction timeline is much faster than building a whole new addition for your ADU unit.

While the process itself certainly adds value to the end game, you’ll then want to consider how much covered parking impacts your ROI. Replacement parking isn’t required for approval of your ADU, so if your house no longer has a private parking space, this could turn away future buyers if you ever decide to sell. In many urban areas, such as Los Angeles, parking is a commodity that boosts home values and stands out to potential homebuyers. As a result, the loss of a standard garage may decrease your home’s overall value, offsetting some of the increased property value you would gain with the new ADU’s addition.

Take into careful consideration your specific housing market and your future plans for your property to make the right decision for you.

Garage conversion cost considerations

The size and scope of work will ultimately play the biggest role in determining the cost and timeline of your garage conversion project. Other factors will include the specific construction costs and permitting fees in your geographic area. Additionally, an ADU garage conversion will cost more if you are also:

  • Upgrading the garage to meet codes
  • Performing any structural improvements
  • Upgrading utilities within the space.

Overall, an ADU garage conversion project could end up costing over $100,000 with an average timeline of nine months from start to finish. In order to maximize your investment and ensure a smoother process, it is recommended to bring in a professional in the initial planning stages. As you work through garage ADU designs, start from the foundation and work your way up.

Foundation and flooring

Most standard garages are built on a concrete slab to accommodate the weight of large vehicles, but the individual building codes for a garage and a habitable structure aren’t the same. You will not only need to make sure the slab is thick enough but that it is also made of the proper cement. And if the slab has degraded, it may need to be reinforced by utilizing another structure through a process called underpinning.

A typical garage also sits on an uninsulated concrete slab, so in order to turn this space into a comfortable living space during any season, you’ll want to add proper insulation and create a moisture barrier between the slab and the flooring.

This slab is also usually several inches below the floor level inside the house, so if the floor is sloped toward the garage door or a floor drain, you may want to level it in order to keep water from seeping in. Adding a curb to the bottom of the garage door opening can prevent water from coming in and protect the framing from moisture.

Walls, windows, and doors

Similar to the floors in your garage not having insulation, its walls likely don’t either. Adding airtight insulation here is crucial to creating a livable space. This may require the need for additional ceiling joists to support the new insulation and drywall. And while you’re in there, check for asbestos.

When deciding on your garage ADU designs, consider how and where you are going to install windows and doors for natural lighting and accessibility. When thinking about the functionality of the space, you may also need to add internal partitioning to add privacy and separation between the bedroom and bathroom.

The last thing to think about is the garage door itself — something many homeowners struggle with. For starters, consider its purpose. Do you want this area to help you incorporate more natural light? Serve as an entry or outdoor access? Be a canvas for art or decor?

If you do decide to remove it and create another wall, you’ll want to make sure the siding blends seamlessly with the rest of the garage so it doesn’t look out of place. If your outdoor space allows, consider installing a patio door for a private entry or outdoor living addition, or consider a large bay window as the perfect compromise between either option.


If your garage isn’t already equipped with plumbing or electrical, adding these utilities will likely be the most expensive upgrades you will have to tackle. Even if local authorities don’t require a separate water or sewage disposal system for your ADU, there are a number of things you’ll have to consider.

  • Does the garage have hookups or service for electrical and plumbing already?
  • How easily can these utilities be routed from the city or utility company to the garage?
  • Is there a sewer line?

In many cases, homeowners are able to connect water, gas, and electric utilities to the existing meter in a house. However, if you are renting the space out to a tenant, you may want to install separate meters for the ADU to monitor this usage exclusively.

In terms of lighting, most garages have a single lighting circuit, and most rooms, especially the kitchen, will require more than that. So depending on the purpose of the space, you may need to add new circuits.

Last, but certainly not least, is plumbing. A water line is likely to be the biggest utility expense because while getting water supplied to the garage may be easy, the drainage aspect could present major problems. Consult a plumber to learn all your options. If you have a laundry/utility room adjacent to the garage, you may luck out. This space could be converted into a bathroom to eliminate the need for additional plumbing lines.

Garage ADU designs and requirements

The ADU designs and requirements in California will slightly differ depending on whether the garage you are converting is attached or detached from the house. General information is as follows:

  • ADUs attached to the home must be set back four feet behind the property line. These guidelines do not apply if you are converting a detached garage or building a new structure altogether. A detached ADU can be a max of 1,200 square feet.
  • An ADU’s bathroom must be at least 30 square feet, and include a toilet, sink, shower stall or tub. It must also include a kitchen that measures at least 50 square feet and a living room/bedroom with a minimum of 70 square feet.
  • Building materials are also regulated, and in the case of ADU construction, you may not use single- piece composite, laminates, or interlocked metal sheathing for the siding. Wooden shingles are also not allowed for roofing.

For more information, view the California State Law guidelines for garage conversion ADUs on single-family properties.

Conversion designs and costs

There are a couple of different levels of designs and costs associated with ADU garage conversions, depending on what you want to achieve and how much you’d like to spend. Like any home renovation, you’ll want to find the best financing option to make both possible.

If you’re looking to complete an ADU garage conversion in the shortest timeline and lowest cost (i.e., $80k+), the best approach is to leave the existing garage structure 100% intact. In other words, no additional square footage will be added, and the overall structure of the garage will remain unchanged — no moving or removing walls. In this scenario, you can still achieve a great functional living space by adding an efficiency kitchen, a small bathroom, and a mini-split HVAC system for heating and cooling, as well as making sure the electrical is brought up to code for a living unit.

If you’re looking for an affordable conversion that offers more versatility and privacy, a larger kitchen and full bathroom will have you coming in at around $120K. At this cost level, you would have individually metered electrical and gas services so that the ADU can be billed separately as tenant utilities. Your contractor could also alter the roof structure to be vaulted in shape, creating a lofted bedroom for added privacy from the main living area.

The final level involves a full custom rebuild, starting at $175k and would include the demolition of the existing garage to rebuild a much larger floor plan or a second story addition. It can also have multiple living and sleeping areas to accommodate more residents. When all is said and done, the transformation of the ADU will leave no signs that it was ever built initially as a garage, giving you a brand new addition that elevates your home and offers a new purpose seamlessly.

If you’re interested in learning more about garage ADU designs or want to find out how our renovation loans can help finance your garage conversion, contact RenoFi today.

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