Garage to ADU Conversion: 16 Questions and Steps to Consider

Garage to ADU Conversion- GMJ Construction Inc
Last Updated: June 18th, 2025

Published on

July 14, 2024

Garage to ADU conversions provide extra living space for the homeowners. ADUs are separate, independent, standalone living units that are legally rentable. The ADU (Additional Dwelling Unit) is a separate tiny house, making the lifestyle nomadic and straightforward. If you have an unused garage, converting it into an ADU would be a wiser choice.

Before getting started with an ADU conversion, you should consider the following steps and get answers to the below questions. Suppose you want your conversion to be done seamlessly and on time; then hiring a good contractor will make a significant portion of your job easier. Timely permit clearance and practical cost analysis will allow you to construct your dream ADU.

Looking to stretch your renovation dollars? Check out our tips on renovating your home on a budget

You must know that it is always crucial to seek the help of a reputable construction company in the Bay Area, such as GMJ Construction Inc. We have the best ADU conversion experts who can guide every step. 

However, below are some must-know steps to consider, along with informational FAQs regarding a garage to ADU conversion. With no further ado, let’s get started.

1. Upgradations Done for Converting a Garage to an ADU?

Upgradations needed for Garage to an ADU - GMJ

Even if your garage is in brand-new condition, built just a year ago, you will still need to make a couple of upgrades to convert it to an ADU. It would also be helpful because, in some communities, residential building codes differ from those for garages. We are sharing the essential upgrades required with you so that you can have a rough idea of the whole process, and no one can overcharge you. The basic upgrades that you would need to do are as follows:

  • A new ceiling will replace the existing one, helping to support the drywall and insulation.
  • Putting a moisture barrier between the slabs and the flooring.
  • Upgrading the garage by installing plumbing outlets.
  • Windows and doors placement, exterior finishing, and interior amenities installation.

2. Should I Prioritize a Garage for an ADU Conversion Over a JADU Conversion?

The JADU (Junior Accessory Dwelling Unit) is a smaller version of an ADU, a standalone dwelling unit that can function independently. ADU and JADU must have their separate entrance and kitchens. A JADU can share a bathroom with the existing home, while an ADU must have its separate bathroom.

ADU Types - GMJ Construction

Considering the distinction between an ADU and a JADU, we recommend prioritizing an ADU over a JADU. Similar to a separate independent living space, an ADU features its own utility bathroom, which enhances its value compared to a JADU. As a result, the chances of renting an ADU increased.

3. Garage to ADU Conversions: What Is the Total Cost?

One of the best investments in maximizing apartment space and achieving a high rate of return is converting a garage to an ADU. Converting a 400 sq. ft. garage to an ADU will cost approximately $50,000. So, if you have a garage covering a larger square footage area, you will pay more. But don’t worry; whatever you spend at this stage will generate significant revenue in the long run.

Total Cost of Garage to ADU Conversions - GMJ Construction

The reasons why a garage to ADU conversion will cost a bit more are:

  • You are removing all the HVAC, water, and electrical extensions from the main home to the garage. Then their installation and quality operation make the process expensive.
  • Depending on the size of your garage-covered area, if it’s larger, it will require more work and materials, which will increase the cost.
  • Clearing out the permit to design the entire ADU will cost you a significant amount. So, handing over the job to the contractor will make you hassle-free, but it will add an extra grand to the bill.
  • Additional foundation laying, quality insulation, and appropriate roofing make up the final budget.
  • Interior designing, flooring, paneling, and all sorts of final finishing will all depend upon your requirements. Therefore, the more spacious you make your ADU, the higher the conversion cost will be.

4. Garage to ADU Conversions: What’s the Timeline?

If you are ready with the conversion clearance and have a good garage foundation in good condition, a garage-to-ADU conversion will take around 4-6 months. Your conversion can be even faster if you hire a reputable contractor like GMJ Construction. Several key elements will determine the timeframe for your garage-to-ADU conversion. A few of the most important are mentioned below.

The Condition of Your Existing Garage:

Some garages are well constructed and have a strong foundation with good-quality framing and roofing. For such garages, there would be no need to reconstruct the framing or relay the foundation. As major work will already be done, more time will be saved, and your ADU conversion will be completed in a short period.

The Dimensions of Your Existing Garage:

If you have a small garage covering a smaller area, it will require less work. A small garage conversion will cost less and save time than a larger garage. For a garage to be converted into an Accessory Dwelling Unit (ADU), it must be larger than 150 square feet.

Status of Utility or Code Permit

Any unpermitted garage will require you to obtain a permit first, which will take time. If your garage is already permitted and you get a permit from your HOA (Homeowners Association) for the conversion, it will save a significant amount of time. And your garage-to-ADU conversion will take less time.

Availability of materials and type of contractor

Handing your garage over to an ADU conversion to a responsible and experienced contractor can make many things easier. Additionally, they will complete the conversion in a shorter timeframe. A reputable contractor like GMJ Construction, Inc. can assist you with the design and material selection process through their innovative design team. So, time conversion will also depend on the type of contractor.

5. Can I Convert an Illegal Garage to an ADU?

We discourage any construction in an illegal, unpermitted garage. It would be best to opt for construction and conversions only on a legally permitted site. If, for any reason, your garage or site is not allowed by the HOA and you wish to legalize it before commencing any work, please proceed to the next step. You will find it helpful.

6. Are there Ways to legalize an Unpermitted Garage?

If your garage is unlicensed and not permitted, the best practice is to comply with all the legal requirements and remove all illegal items. For example, if you have a garage with a window over it and having a garage with a window is not permitted, then replace the window with an actual wall to fill the window space.

We recommend hiring an expert to inspect your garage. Then, acting as a middleman, he will seek out the laws of your HOA and enlist all those who have disobeyed them. Then he will share with you the possible ways to comply with the regulations. As a result, you will legalize your garage and make it ready for an ADU conversion.

7. What to Do If Power Lines Pass Over Your Garage?

Power lines require an encroachment permit, which can be obtained by submitting a written request to the service provider. As a result, depending upon the company, they will leverage you to construct an ADU by remaining 5-10 ft lower than the actual lines.

8. Can a Granny Flat Be Built Above My Garage?

Most homeowners opt to build a granny flat over their garage to maximize a property’s potential and increase its value. Building up a granny flat is much more complicated than it might seem.

To build a granny flat over your garage, you must also make the exterior uniform. The granny flat shouldn’t appear as a building out of the box. If the newer granny flat looks out of place, it will decrease your property value and act as an unwelcoming gesture.

Building a granny flat will require more money. As a granny flat is a heightened structure, one must ensure its strength to withstand the load. Additional flooring and roofing would be necessary to construct the extra infrastructure above the garage.

9. Is It Worth Demolishing the Old Garage and Constructing a New ADU?

Considering the budget and build cost, constructing an ADU over an existing garage is more cost-effective. However, you must still consider the condition of the garage. If your garage is under 50 years old, has a good foundation, and the framing is in good condition, then you can consider an ADU conversion using an old garage. Alternatively, if your garage is older than 50 years, we recommend demolishing the old garage and constructing an ADU from scratch. An expert will test the framing and foundation quality and let you know whether building a new garage will be extraneous or valuable.

10. Is It Necessary to Reconstruct the Garage Foundation?

As most garages are built over slabs, it’s necessary to reconstruct the foundation. You can do it by applying a layer of concrete above the slabs, using a moisture barrier between them if your slabs are in good condition. If the slabs are cracked and in poor condition, then underpinning by digging a trench and installing a new foundation is recommended.

A strong and rigid foundation can eventually give rise to a robust and earthquake-resistant building structure, so you must pay attention to foundations while doing a garage to ADU conversion.

11. Can I Add Separate Utilities To ADU?

Yes, you can add special utilities to your ADU and install separate utility meters, which will help track consumption if you rent out the ADU. Establishing different water and power meters will cost you a couple of additional dollars, such as $ 3,000–$4,000, depending on your utility provider. Adding separate utilities will make you feel like you’ve saved money, but eventually, when renting out both the ADU and your existing home, you’ll be paying for them. It will cause difficulty for both tenants regarding the payment of the bill.

12. Does ADU conversion also require the construction of a separate parking lot?

No, there is no such rule requiring a separate parking space for an ADU. For the sake of the requirement, if you have extra space, you can allocate some for the individual parking of the ADU. It will encourage more buyers to prioritize it over an ADU with no parking.

13. Are Loans Given for Garage to ADU Conversions?

You can get easy, hassle-free loans with the lowest interest rates for ADU Conversions. To obtain a loan, you must be prepared with your home documents, employment history, and credit card information. Some essential loans that you can get for ADU conversions include:

Cash-out refinances of mortgage loans:

This cash comes with no strings attached, and even the interest rates are lower. You can obtain an amount of cash equivalent to the percentage of the home’s current market value minus the outstanding mortgage balance.

Renovations, remodels, and construction loans:

In this loan category, you can get all the conversions fully funded by the loan. To get these loans, you would need a solid bid from a contractor. The loan lender will evaluate the future value of the ADU and give the loan based on that. These loans fully fund the conversions, so they have higher interest rates.

14. What Are the Property Taxes for Converting a Garage to an ADU?

Whenever the property rate is increased by 1%, the tax rates for ADU conversions are also adjusted accordingly. For renting out an ADU, the total property tax will depend solely on the construction expenses, as the city evaluation indicates that the garage was already present with the main house.

15. Can Both the ADU and the Actual Home Have Two Different Addresses?

Depending upon your state law and jurisdiction system, some countries will automatically consider an ADU a separate independent unit and allocate it a different address. While obtaining an individual address is optional, it’s up to you to decide if you need one. You can write to the local judiciary. In some states, the ADU and the existing home are given the same address.

16. Is Garage to ADU Conversion Worth It?

ADU Conversions are worth the money. They not only add extra living space to the existing home but also increase the property’s actual market value. But in time, this living space can be rented. ADU conversions are the best way to create a separate, independent living space for your family or rent it out. Building an ADU will be worth the investment if you don’t need your garage for vehicles.

Suppose you’re considering converting a garage to an ADU, utilizing a permitted legal garage, to generate extra cash flow. In that case, building an ADU conversion is a worthwhile investment option for you.

Hire GMJ For ADU Build and Conversion in the Bay Area

Converting a garage to an ADU is ideal for creating a small, separate residential unit. ADU can also serve as an excellent guest cottage. An ADU can be used as an independent private office or workspace and can even be rented out for additional income. The process of converting the garage into an ADU is a cost-effective approach. Building an ADU conversion is an ideal way to create a small, separate residential unit within an existing one.

Above, we have shared 16 essential questions and steps to follow if you opt for a garage to ADU conversion. They will be helpful. Starting with a quality foundation in a legally permitted garage and working with an experienced contractor like GMJ Construction Inc. can make your ADU conversion project more accessible. Contact us for a streamlined approach to the best ADU Bay Area construction from the renowned builders at GMJ Construction Inc.