When advertising for professionals in the construction industry, use an RFP (Request for Proposals) or an RFQ (Request for Qualifications).
In the RFP, the professional is asked for information (a bid) on charges - whether charges would be based on price per hour, a percentage price, or a fixed price. In an RFQ, the professional is asked questions about qualifications and expertise, not price.
Regardless of whether you use the RFP or RFQ, the basic and fundamental objective is the same-- hiring the most qualified professional at the best price.