Legal questions are best answered by the attorney you retain.

Don't hesitate to ask them if they're going to build it at 9' or 10'. Start with the person you signed the contract with.

If it's in the contract, that's what rules. If there's a conflict within the contract, there should be an order of precedence clause.

10 ft vs 9 ft is a big issue. I would not want a home with 9' ft ceilings if my intent was to have 10' ceilings.

It's not just studs, drywall, osb and siding. The windows and doors have to be proportional, the stairs will be 2 treads longer and might not fit. Are there exterior elevations of the house on the construction drawings that reflect the designed height of the house?

The other variable is code height. The height of houses is usually limited by the town/city/whatever. Even in rural areas that don't have a lot of rules. Reason? - fire trucks.

Windows is the other really big issue. Do the plans have window sizes on them? Also door height - 9 foot ceiling can live with "normal" size doors but 10 ft cannot.

I am the last person to lawyer up ... but I would strongly consider doing it if you have any serious money committed.

The difference between 9 and 10 ft ceilings is major - like tens of thousands of dollars. This is not a simple oversight by a competent builder. This is a major red flag.

Some people like 10 ft ceilings but I think 9 ft is a better choice. See if you can get a credit for the change.

Agree 100% with what David Cary said. Our current project is a heavy reno of a house with 9' ceilings but with 6'8 head heights for doors and windows, and even with the lower ceiling height 6'8 feels bizarrely low - way too much sheetrock between the top of the windows and the ceilings. In addition to looking weird the rooms are darker as a result. We're pushing all the headers up to 8' in the 9' rooms and 10' in our 11' room.

I find it quite bizarre that details like this are offered as adjustable items in the contract. It makes me wonder if they will in fact scale everything else up, or leave you with something oddball like I just described that you'll be unhappy with. Typically the height of the ceiling is something that is integral to the design of the house as it impacts not only the appearance of the rooms from the inside but also the exterior elevations. If you have an upstairs it's also something you have to think about (10' means going up two more risers to reach the 2nd floor - so many 2 storey houses around here are designed to have their second storeys built over 9' sections, and have living rooms with higher ceilings built outside the footprint of the second storey).