Naturally, any project owner working with an architect needs to have a sense of the potential cost of their planned project from the earliest stages to determine its feasibility. In most cases it is accepted that Order of Magnitude estimates can have varying degrees of accuracy based on the amount of information provided. However there are a number of factors that can yield a much more precise (and useful) estimate. These factors are enumerated below.
It may seem obvious, but the first and most important factor in developing an accurate order of magnitude budget is to select a qualified, professional construction management or general contracting firm to provide it.
There is no shortage of construction companies in business today. Most are very busy with lots of projects in their pipelines and not all of them have the necessary skillsets to develop a truly accurate estimate in the early stages. Be sure to select a construction management firm or general contractor that has both a professional estimating team and the sufficient available resources to devote to your ROM estimate. It is also crucial to choose a firm with recent project experience directly related to the type of project you are considering.
If you are dealing with a qualified firm they should recommend a physical walkthrough of the property. This is a must if the project involves a renovation of an existing building but it may also be helpful even in the case of new construction as it may give insight into the civil engineering/site modifications that may be required. Both of these could significantly impact the cost. There is no substitution for seeing with your own eyes.
By visiting the site, a construction expert will be able to compare the plans (regardless of their level of detail) and the goals of the project with the existing conditions in order to identify challenges, offer suggestions and alternatives that may reduce cost or prevent costly corrections later. Accompanying the estimator during the walkthrough is highly valuable since you will be able to share your overall concept and specific details that may not be communicated clearly in preliminary drawings.
Ideally, the final product of your order of magnitude estimate should be more than a single page with a price. A qualified construction manager will provide not only the overall estimate (with variations) but also attach a comprehensive scope of work (based on the current conditions). This line by line outline of the phases of construction should lay out precisely what can/will be done for the price in the estimate. Of course this will likely evolve as the project develops and it will, of course, increase in details and specificity but by following these principles you can feel secure that you have in fact received an accurate, workable estimate for your planned project.