Ask for the license number on the contract. It should read #1094838 and the business name should match ONESTOP ADU.
Keep the deposit small. For home improvement contracts California caps the down payment at $1,000 or 10% of the price, whichever is less.
Ask who will be on site. This license is exempt from workers' comp, which is only allowed with no employees. If a crew shows up, ask who employs them and how they're insured.
Make sure they pull the permit. The contractor, not you, should apply for the building permit, so the city holds them responsible for the work.
OneStop ADU is listed because its license passed our checks. Listing is free and isn't an endorsement; we don't take part in your contract. Details change, so confirm the license at cslb.ca.gov before you sign.