Topsoil Thickness ?

Hi,
Live near Boston.
Will be having a new lawn put in. Present one is probably about 25 % grass, the rest being very, very scragly with pebbles, rocks, etc.
Question: when I get a contractor in to quote, what should I be asking him regarding the "proper" thickness of topsoil to put down (I guess prior to rolling it all flat) ?
What else should I be asking him ?
e.g., what else is important for a good, complete job ?
Thanks, Bob
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I would ask for 4 inches with lots of organic matter tilled in and rocks and debris raked out. Most will try to tell u 2 inches is enough, especially if this is for sod. Total BS.

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