How to Convert Your Old Garage in an ADU (2025 Guide + Bonus)

Transforming your garage into a warm and stylish Accessory Dwelling Unit (ADU) is a dream that can come true! Here’s your comprehensive 10-step guide to garage conversion ADUs, an increasingly popular living alternative. Whether you’re converting a standalone garage or an attached structure, these reinvented spaces might be ideal for making passive income, entertaining visitors, or creating multigenerational living. We’ll go over zoning restrictions, working with builders, and the specifics of putting out your garage ADU space. Let the learning begin!

1. Check the local zoning and regulations.

Starting strong, the most critical step is to ensure that your local government permits garage conversion ADUs. The last thing you want to do is begin the process only to discover that your aspirations will not be realized due to permit requirements. Begin by calling your local planning or construction department to confirm whether a garage-to-ADU conversion is permitted. For example, while California’s state ADU rules generally enable conversions, some local municipalities may impose size and design limits. Other states may allow ADUs, but they must be detached or cannot be rented out short-term.

When you obtain the green light for zoning, make sure to request a zoning verification letter as you’ll need paperwork if you go to market or face future compliance problems. Furthermore, you must carefully study what your local county allows for the size of the garage ADU, whether the garage can be detached, and what parking restrictions apply. All of these facts will have an impact on your plans in the future, so take the time to complete this step completely.

2. Assess the Garage Condition

The next step in the garage ADU conversion journey is to examine the current garage condition. This may necessitate engaging a local building inspector to conduct a meticulous inspection of your garage. They will check the foundation slab for cracks, the roof for leaks or sagging, and the walls for decay or pest damage. Structural support is an important part in how your garage ADU will turn out. This stage may indicate that you need to completely demolish the garage and rebuild it from the ground up, or replace the foundation. Alternatively, you may find termite damage and unsupported beams, which must be addressed before proceeding.

The list also includes inspecting existing plumbing and electrical for safety and code compliance. If you’re converting a garage, you’ll have to install new wiring, lighting, and plumbing. All of these require current code compliance and installation. In summary:

Hire a local construction inspector for complete peace of mind.
Inspect the structure’s foundation, roof, walls, plumbing, and electrical.
Determine what repairs or upgrades are required to meet building codes.

The video above also provides a detailed description of checking and preparing your garage ADU for the following step.

3. Hire an architect or designer

Planning and design are the most exciting parts, even though zoning regulations and inspections aren’t the most enjoyable. Architecture and design are this step’s two primary constituents.

Buildings
You will need to draft drawings to submit to the city for approval if you are transforming the garage from the ground up. We advise working with a local architect because this could feel daunting. They can help you create blueprints that fit your ideas and local codes. Don’t forget to include thorough layout, utility, window, insulation, and egress plans.

Consider carefully what your ADU will cover and what it will be used for.

Create
As you get closer to the design stage, the options can seem genuinely limitless. Your imagination and vision can truly come to life when designing your garage conversion ADU. Speak with a local designer who can help you with the creative process and provide design advice if you don’t think you’re an artistic person or if design isn’t your forte. Keep in mind your goal and vision for the garage ADU, just as you would when working with the architect. You should install the proper electrical connections for peaceful outlets and lighting if you want to create a relaxing atmosphere. Or, if natural light is your thing, think about installing an additional skylight to allow in that gorgeous sunlight.

4. Getting the permits

After finalizing and preparing your blueprints, they must be submitted to the city for approval. This process can be quite lengthy, taking several weeks or even months. This allotted time provides an opportunity for further planning and envisioning of your desired space.

The permits required by your local municipality will typically include electrical, plumbing, mechanical, and energy permits. For instance, in California, the necessary permits for a garage conversion ADU are outlined below. Do note, however, that the requirements may differ depending on the state in which you reside. In order to ensure full understanding, it is advisable to reach out to your local building department. Should you choose to invest in a modular ADU from Zook Cabins, we can offer guidance on obtaining permits.

Permit Type Approximate Cost (CA) Reason for Permit
Building $3,000-8,000 Encompasses structural and framing work
Plumbing/Electrical Included in the building permit cost Establishes legality of wiring and plumbing changes
Mechanical/Utility Exempt for structures under 750 square feet Mandatory for larger ADUs requiring water hookups and sewage line connections
Energy Compliance (Title 24) $800-1500 Compulsory for all conditioned spaces in California

5. Make Structural Improvements

Bolstering the Foundation

The initial step in converting your garage into a livable space is building a sturdy foundation that can support additional structural loads. Many garages have shallow slabs that are only designed to hold the weight of vehicles, not framed walls, insulation, and drywall. If your garage falls into this category, you will need to undergo foundation underpinning. This may involve digging a trench around the perimeter, setting deeper rebar, and pouring new reinforced concrete footings. Alternatively, your contractor may opt to pour an entirely new concrete slab over the existing one.

Increasing the Ceiling Height

The standard ceiling height in a garage is typically 8-9 feet, which barely meets certain state requirements for Accessory Dwelling Units (California’s minimum is 7’6″), and most homeowners prefer 9-10 feet for a more spacious feel. Raising the ceiling often requires installing new ceiling joists or restructuring roof rafters. Joist ties that are obstructing construction may need to be removed, or the entire roofline may need to be reframed. This will lift the ceiling joists into the attic space, creating a taller ceiling height. It is crucial to carefully evaluate which joists can be removed, and hiring a structural engineer is the best course of action. Overall, increasing the ceiling height has a significant impact on the upgrade of your garage ADU. It will greatly improve the comfort and resale value, while also meeting building regulations.

Replacing the Garage Door

One of the final yet crucial structural changes needed for your garage is replacing the garage door. This is essential for transforming the ADU into a home, rather than a confined garage. You can opt to replace the garage door with a framed wall or add additional natural light by incorporating floor-to-ceiling windows and doors. Regardless of your choice, it is vital to ensure that the doors and windows are properly insulated and securely supported. Watch the video above for a detailed walkthrough on transforming a garage door into a framed wall.

Rough-In Plumbing and Electrical for Your Garage ADU

Once you have ensured the structural integrity of your garage, it is time to focus on the plumbing and electrical work. These areas require expertise, so if you are not well-versed in these areas, it is wise to hire a professional who can handle the complex details on your behalf.

Plumbing

For garages without existing plumbing, installing new water and sewer lines is a crucial and often intricate step. The process begins by digging a pathway from the main utility line of your house, or for detached garage ADUs, directly to the city line. The water lines typically include access to hot and cold water, drainage, and proper venting to maintain flow. According to Better Place Design & Build, rough-in plumbing for detached ADUs usually costs between $10,000 to $25,000, with additional fees for sewer and trenching depending on depth and distance. If your garage is an attached ADU, the costs will be lower ($8,000 to $20,000) because you can connect the lines to your existing plumbing.

Electrical

Almost all ADU garage conversions require upgrades to the electrical work to accommodate the installation of HVAC systems and appliances (not typically found in traditional garages). You will need a 100-200 amp service and must determine the best way to connect the electric. Similar to plumbing, you can connect the wiring to your existing electrical meter or install a new panel (required in some states). The video above provides a walkthrough of an electrical rough-in for an ADU, and an experienced local electrician can offer further guidance specifically tailored to your ADU garage conversion project.

6. Insulation and Weatherproofing Tips for Your ADU Garage Conversion

To maintain a comfortable temperature in your ADU garage conversion throughout the year, it is crucial to properly insulate all the walls, floors, and ceilings in accordance with energy code requirements. Utilizing high-quality insulation, especially around energy-efficient windows and door frames, is a key factor for a successful conversion. Most ADU garage conversions recommend using R-13 to R-21 insulation for walls, and R-30 or higher for ceilings, depending on your climate zone. Additionally, ensure the installation of vapor barriers and seal any air gaps with spray foam, while also opting for energy-efficient windows and doors. Adequate insulation not only keeps your ADU warm and cozy, but also guarantees a successful completion of final energy inspections. Choose a modular ADU from Zook Cabins to benefit from top-notch insulation and ProVia energy-efficient windows and doors, ensuring unparalleled comfort.

7. Install HVAC and Ventilation

Alongside insulation, you desire a cooling and heating system that will provide optimal comfort for you and your guests. A recommended option is a mini-split system, offering both heating and cooling capabilities with the added perk of zoned temperature control. Alternatively, you could connect the garage ADU to your existing HVAC system or opt to have an electric baseboard. Regardless, make sure to prioritize proper ventilation in areas like the bathroom and kitchen, as this greatly reduces the likelihood of moisture and mold problems.

8. Finish Interior

Before obtaining the living permit, it is imperative that you finalize all the aspects that will transform your garage conversion ADU into a cozy, livable home. From drywall and flooring to appliances and cabinetry, and culminating in a fresh coat of paint, it’s time to elevate this space into a true residence.

Interior ADU Tasks and Approximate Costs
Task Materials Estimated Cost
Plank flooring Luxury Vinyl Plank (~$2-5/sq ft) $5-10/sq ft (including installation) = $5,000-10,000 for a 1,000 sq ft ADU
Baseboards and trim MDF or pine molding (~$1-3/linear foot) $500-$1500
Drywall ½” Drywall ($12 per sheet) + joint compound/tape $1.50-2.50/sq ft + $3,000-5,000
Cabinets and countertops Stock cabinets, Laminate countertops $3,000-7,000 (ranging from budget to semi-custom)
Kitchen appliances Appliance package $2,500-5,000 (bundled price)
Lighting and electrical fixtures LED Flush Mounts, switches, outlets, panels $2,000-4,000 (materials and basic installation)
Bathroom appliances Shower, toilet, faucets, sink $2,000-4,500
Bathroom tile installation Ceramic Tile $2-$30 per square foot (depending on material) = $400-$5000 (including installation)
Interior paint Top-quality Interior Paint $500-$1200 (for 10-15 gallons)
Smoke/CO detectors Battery-powered detectors $30-$60

9. Certificate of Occupancy and Final Inspections

Inspection of the Garage ADU
You’re nearly there! Obtaining your Certificate of Occupancy, which guarantees that you are able to rent and inhabit the garage ADU conversion lawfully, is the final important step before moving in or renting it out. The procedures for obtaining a CO will differ from state to state, but generally speaking, you will need to have multiple scheduled inspections from your local building department. They will inspect for fire dangers, structural safety, and adherence to local zoning regulations.

10. Is it less expensive to convert my garage into an ADU rather than buy one?

To be honest, it depends. Even while garage conversion ADUs are typically less expensive, there are situations when your garage may be in such poor condition that you may have to start over. Complete excavation, a costly and intricate procedure, may be necessary in some situations. You can be confident that everything will be constructed to meet or beyond code when you work with L Square Construction. This will guarantee a seamless building procedure free of surprises from beginning to end. A skilled group of professionals handles the design, delivery, and setup, so you can relax and take pleasure in the process.