Estimating time and budget is hard and influenced largely by choices that you (the client) will make driven by your preferences for materials and objects that resonate with you. As your design resource I will be a sounding board, a technical resource and a creative influence to translate your preferences and needs wisely and economically into a set of choices that achieve a particular effect within a particular budget. As your builder I will act as your agent to help you achieve the design at the best possible price and in the shortest possible time. These are two separate roles. The service I offer is called design-build for this reason. It is the conjunction of two services and there are many reasons why this is a great idea.
Designers untethered to what is feasible, practical and economical. Having at the contractor's input early on in design can often save money. Having such input continuously because both services are conjoined end to end always does. Exhaustive design documentation in order to achieve accurate pricing is less of an issue. Typically in order to eliminate ambiguity to the point that the contractor is comfortable pricing a design takes time and expense in design phase. Short cuts on design documentation save money in design that is typically spent in contract.