When homeowners begin exploring the possibility of adding an accessory dwelling unit, one of the first questions they often ask is whether they can build over an existing garage.
Keeping the current garage may initially seem like the more affordable and efficient option. After all, part of the structure is already there. However, once the condition of the garage, its foundation, current building codes and the work required to support a second story are considered, retrofitting does not always provide the savings homeowners expect.
According to Matteo Carpani, Operations Manager at CASK Construction, rebuilding is frequently the more practical financial decision, particularly when the existing garage is older or detached from the main house.
“The biggest misconception about building an ADU on top of an existing garage is that there is going to be a savings in money or time,” Matteo says. “In reality, that does not usually happen.”
Why Retrofitting May Not Save as Much as Expected
An existing garage was not necessarily designed to support an additional floor of living space. Before an ADU can be constructed above it, the garage, foundation and footers must be evaluated to determine whether they can carry the additional weight.
If they cannot, the structure may require extensive reinforcement.
“Building on top of an existing garage requires a lot of construction surgery,” Matteo explains. “The footers and the structure may need to be retrofitted to support the weight of the ADU.”
This work can include carefully removing portions of the existing structure, cutting into the slab, installing new footers and building an independent support system within the garage. In some situations, homeowners can lose usable square footage because new structural elements must be added inside the existing walls.
Unlike a straightforward demolition, this type of work must be performed carefully to avoid damaging the portions of the garage that will remain.
“A quick demolition is less complicated than a surgical demolition, where we have to carefully remove roofing, wiring, doors and other parts without interfering with the structure,” Matteo says.
By the time those specialized costs are added, the financial advantage of keeping the original garage may be minimal.
“The savings are often very small, and you still have an old garage underneath the new structure,” Matteo says.
The Age and Construction of the Garage Matter
Not every garage is a poor candidate for renovation. The age, material and condition of the structure all influence what is possible.
A relatively new concrete-block garage may be easier to reinforce than an older wood-framed structure. Original building plans can also help engineers understand the size and condition of the existing foundation and footers.
“If the garage is less than 10 years old, constructed from block and we have plans showing how it was built, it may be worth considering a retrofit,” Matteo explains.
As the structure gets older, uncertainty increases. Wood-framed garages may have deterioration, moisture intrusion or termite damage that is not immediately visible. Older garages may also have been built under significantly different code requirements.
“If it is more than 10 or 15 years old, particularly if it is wood, we have to carefully examine the condition,” Matteo says. “At some point, it may no longer be worth keeping.”
Even when an older garage can technically be reinforced, homeowners must consider whether it makes sense to invest heavily in a structure that will remain partially old.
“The extra money is being spent, but you still end up with an old structure rather than a completely new one,” Matteo says.
Retrofitting Does Not Always Shorten the Timeline
Homeowners may also assume that preserving the garage will allow construction to move faster. However, retrofitting often requires specialty work that can make the schedule similar to—or longer than—a complete rebuild.
“The timeline is often similar to a new build, even when demolition is included,” Matteo says. “Sometimes retrofitting can take longer because we have to work under the existing footer, modify the structure or retrofit utilities.”
Crews must work around existing walls, foundations and utility locations. Unknown conditions may not be discovered until construction begins, which can create additional work, cost and scheduling changes.
With a new structure, the construction team has more control from the beginning.
“When we build new, everything is easier to understand,” Matteo says. “We know what footer we are installing, how the walls are being built and that everything will meet current code.”
When Does Keeping the Existing Garage Make Sense?
Although Matteo frequently recommends rebuilding a detached garage, there are circumstances where preserving the structure may be worthwhile.
One consideration is whether the property is located within a historic district. Historic regulations may limit or prohibit demolition, making renovation the necessary path.
Existing setbacks can also affect the decision. An older garage may have been built closer to a property line than current regulations allow. Demolishing it could mean that the replacement structure must be moved inward, resulting in the loss of valuable yard space.
“You do not normally retrofit for the savings,” Matteo explains. “You may do it because there is a space constraint, because you want to preserve an existing setback or because historic regulations do not allow the structure to be removed.”
An attached garage may also require a different evaluation because it is integrated into the main house. In that situation, preserving and modifying the garage may be more reasonable than removing a portion of the home.
“The biggest factors are whether the building can legally be demolished and whether it is fairly new, built from block or located within a setback that the homeowner does not want to lose,” Matteo says.
The Long-Term Advantage of Starting New
For a detached garage that is older or requires substantial structural work, demolition and rebuilding can provide homeowners with greater certainty and a more valuable finished product.
Starting with a cleared site allows the foundation, walls, plumbing, electrical systems and drainage to be designed specifically for the new garage and ADU. The entire structure can be built to current codes rather than combining new living space with an aging structure below it.
“If the garage is standalone, I would usually demolish it and build a stronger structure that is completely up to code,” Matteo says.
A new structure can also reduce the likelihood of discovering hidden damage after construction begins. While every construction project includes some uncertainty, detailed site investigation and pre-construction planning can identify many potential complications early.
“The long-term value is having a structure that is brand new,” Matteo says. “You get a better product, and there is more certainty surrounding the timeline and potential surprises.”
Planning Is the Most Important First Step
Whether a homeowner ultimately chooses to renovate or rebuild, the decision should be made only after the property and existing structure have been thoroughly evaluated.
The pre-construction process may include reviewing zoning and setback requirements, researching historic restrictions, locating utilities, evaluating drainage, examining the existing foundation and consulting with engineers and trade professionals.
These early investigations help the project team develop an accurate scope and reduce the chance of unexpected costs during construction.
“Building the ADU itself has a cost, but there are many other pieces that need to be investigated,” Matteo says. “The configuration of the land, utilities, drainage, landscaping and connections to the existing property can all affect the final price.”
At CASK Construction, the goal of pre-construction is to identify these considerations before homeowners invest heavily in construction.
“Construction becomes enjoyable when you plan ahead,” Matteo says. “It can become stressful when you do not have a plan. Our job is to understand the potential problems before construction begins.”
By taking the time to compare the true cost of retrofitting with the cost and long-term value of rebuilding, homeowners can make a more informed decision about their property.
In many cases, starting over does not simply create a new garage and ADU. It creates a stronger, more predictable and more valuable structure designed specifically for the homeowner’s future.
Not sure whether renovating your existing garage or starting fresh is the right choice for your property? CASK Construction can help you evaluate the condition of the structure, local requirements, potential costs and long-term value of each option. Reach out to our team to discuss your goals and take the first step toward a plan built around your property.
