I'm a real estate agent building a $480,000 backyard home for my mom. I still made these costly mistakes.

At first, Bonjour thought she had found the right architect for the project. They designed the ADU and even promised to guide her through the permitting and construction process. But they didn't follow through.

Bonjour said hiring the wrong person was one of the "biggest mistakes" she's made during the project.

Although she found someone to take over the project after interviewing about 10 builders, mistakes made by the original architect have made construction more difficult.

"Watching my builder try to translate that original designer's plans is like trying to fit a square peg into a round hole," she said. "It's basically like he was working off somebody else's recipe."

Every city has its own rules for building ADUs, and some are more complicated than others.

In her part of the Coachella Valley, Bonjour had to secure permits for the roof, electrical work, plumbing, HVAC, and solar panel installation.

"The city was pretty tough on solar," Bonjour said. "The first two electrical inspections were fine, and then on the third inspection, they determined we needed solar panels. Getting solar here in the desert is its own job. Researching the proper solar companies and their packages was another."

While the family waits for the ADU to be finished, Bonjour, her husband, their son, and her mother are all living in the main house. It's been difficult sharing space, especially the kitchen.

"My mom loves to cook, so having her own kitchen will be really important to her," she said. "She was making a recipe and said, "It'll be different when I have my own stuff." I think the whole thing is bittersweet, but once she sees her finished kitchen, she'll feel different."

Construction has caused headaches for the cul-de-sac, too. Because the neighborhood's electrical transformer is outdated and overloaded, the local utility needs to upgrade it before the ADU can be connected. The work has involved digging up Bonjour's driveway and those of two neighbors.

"Our neighbors had to deal with the inconvenience of their driveways being out for a week and a half," she said. "My contractor and I are trying to be as considerate as we can and get the best people to do the work as quickly as possible. Thankfully, the neighbors have been gracious and nice along the way."

Bonjour's family budgeted $480,000 for the construction of the ADU. Of that total, they set aside $100,000 as a contingency fund.

"As a realtor, and as someone who's always telling people to ask all the questions, I also hate to be caught by surprise," Bonjour said. "With the contingency budget, you build in extra money that you hope you won't need. It's something everyone should consider, whether they're building a million-dollar home or a small casita in the backyard."

Unfortunately, unexpected costs, like hiring replacement contractors and installing a new electrical panel and solar panels, have exhausted the reserve. Bonjour said the extra expenses have trickled into the family's day-to-day life, making it harder to justify things like family trips and upgrading the main home.

"I wish I had allocated it differently, but again, I think that planning with the right people at the onset would have saved so much headache and time," she said. "We'll have to wait on upgrading the home. That's kind of annoying, but that's where we're at. Lesson learned on that one."

If she had to do it all over again, Bonjour says she would make different decisions.

"I'm a professional, so I hold myself to a higher standard of asking questions and being informed," she said. "I like to have good relationships with the builders, contractors, and vendors. In some ways, I feel like I should have known or seen what was going wrong, and that becomes really hard to kind of wrestle with."

She still tries to give herself grace. For all its challenges, she believes building the ADU has been the right decision.

"We know that having Mom in proximity is right for our family," she added. "We've learned some lessons along the way as homeowners, as a realtor, as a child, as a mom who's growing old herself."