FAR 36.203 requires an independent government estimate for construction contracts "at the earliest practicable time". Conducting a search elsewhere in the FAR, DFARS and DFARS PGI does not yield any results as to mandating when this estimate is required, but it is inferred through FAR 1.108(c), one of the FAR conventions. "Unless otherwise specified, a specific dollar threshold for the purpose of applicability is the final anticipated dollar value of the action, including the dollar value of all options." The dollar threshold of the acquisition needs to be known, even if it is under the SAT. Dollar thresholds are tied to certain approvals, actions and terms and conditions of the solicitation and resulting contract.
A recommended website to put together independent government estimates is at https://hallways.cap.gsa.gov/app/#/igce. This GSA website provides an IGCE tool that takes into account the different costs that need to be considered when creating the anticipated dollar value of the contract.