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

Shalini Karnani Bonjour in her California backyard.

Courtesy of Shalini Karnani Bonjour Shalini Bonjour is building a 1,100-square-foot ADU in her backyard so her mother can age in place.

The project is taking longer than she expected and is going over budget.

Bonjour shared the five biggest lessons she's learned during the process with Business Insider.

After her father died in 2024, real estate agent Shalini Karnani Bonjour moved her mother from Michigan to California to be closer to her and her family.

"My husband and I didn't feel comfortable having her so far away, especially when it came to her day-to-day needs," Bonjour, 50, told Business Insider.

Later that year, the family bought a home with a backyard large enough to build an accessory dwelling unit for her mother.

In 2025, they broke ground on a 1,100-square-foot ADU.

But what was expected to be a six-week project has dragged on for months and cost the family thousands more than expected.

"The whole process has been difficult because there are so many things to manage," Bonjour said.

"You're trying to keep the project affordable, keep your neighbors happy, and stay on track.

I can only imagine how overwhelming this would feel for someone without any insight into the process, because it is a huge undertaking." Here are five hard lessons Bonjour has learned from the project, from choosing the right contractor to budgeting enough money for unexpected challenges.

A bad contractor can make or break a project The ADU during the early stages of its construction.

Courtesy of Shalini Karnani Bonjour At first, Bonjour thought she had found the right architect for the project.