My husband and I just signed a contract and the builder is placing a lien on our lot just in case we do not pay on time, is this legal?
Short answer: No. A mechanic's lien is not proper unless there is debt owed for construction improvements that have already occurred.
If the property is the owner's homestead (meaning that the owner owns the property and lives there) then your contract with the builder would have to satisfy the requirements of Texas Property Code sections 53.254, et seq., which requires specified homestead lien warnings, signing by husband and wife, and recordation in the county property records.
You really should retain a construction attorney to review and evaluate your legal position based on the contract and pertinent documents. The right to be entitled to file a mechanic's lien is set out in Texas Property Code Chapter 53, and you really need to make sure that if the builder does file a lien, the builder has properly and appropriately done so.
Filing an invalid lien could subject the filer to liability under the Texas Fraudulent Lien Act, and the filer could be liable for statutory damages of $10,000, or actual damages, whichever is greater, plus attorney's fees.
Good luck.