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

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.