Legal Question in Construction Law in California

That depends on the details of your contract and the terms of the non-compete.

It's "legal" in the sense that putting such a clause in your contract doesn't break any laws. On the other hand, there's a real question as to whether it's enforceable. California law strongly favors competition, and non-compete clauses are often held by courts to be unenforceable because they are anti-competitive. That's why the details of your contract and the terms of the non-compete clause are so important. Another factor is that if you decided to do something that violated the non-compete clause and got sued, even if you won on the ground that the clause was contrary to public policy and hence not enforceable, you'd have spent a lot of time and money on your defense. (If there's an attorney-fee clause in the contract, that's kinda like "double or nothing" because you'd pay their fees if you lost.