I am a builder and have a homeowner in a new home with a concern about bubbles in the first floor toilet when the upstairs washer runs the rinse cycle. My plumber has gone under the house and moved the tie in for the washer as far as possible form the toilet. He has also inspected the lines with a camera (with water running) and confimred that there is no obstruction in the sewer line all the way to the street. In all comparable plans in the same neighborhood that are plumbed the same way we do not have the same problem. I don't want to dismiss the issue if it is a legitimate concern. However, nothing has backed up, overflowed and functioned improperly - but for the bubbles. I welcome any thoughts or suggestions on this. Thanks.

It usually happens when there is a cloggage developing in the main sewer, down the slope from the toilet. Also, if toilet lacks proper venting, passing water from washer will cause water in toilet to move up and down.

I am a builder and have a homeowner in a new home with a concern about bubbles in the first floor toilet when the upstairs washer runs the rinse cycle. My plumber has gone under the house and moved the tie in for the washer as far as possible form the toilet. He has also inspected the lines with a camera (with water running) and confimred that there is no obstruction in the sewer line all the way to the street. In all comparable plans in the same neighborhood that are plumbed the same way we do not have the same problem. I don't want to dismiss the issue if it is a legitimate concern. However, nothing has backed up, overflowed and functioned improperly - but for the bubbles. I welcome any thoughts or suggestions on this. Thanks.

Milos correct. Bubbles indicate backpressure. Since you've inspected the lines and found then clean, and, if it only happens when the washer discharges I suspect something in the drainage design. A elbo placed where when the force of the discharge hits it it sends a momentary puff of air back up the line where you see it in the form of bubbles. In that scenario everything would drain and the pipes would camera as clean with no clogs.
I guess you could run a dedicated vent on the toilet to exhaust the backpressure but the easiest way would be to educate the owner. This does not sound like a serious problem. Goodluck, tom