Phase I Environmental Site Assessment is usually required by banks during commercial property transactions. Basically a building inspection but aimed towards environmental issues (i.e., spills, petroleum, asbestos, etc). There's deed research and interviews performed, as well. This is usually a day or two in the field and the same in the office. Review the ASTM guidelines for requirements.
I've never done an environmental impact statement (EIS), but from my understanding, they are required during the design phase of project with sensitive areas (i.e., wetlands, oceans, lakes, etc.). For example, Giants Stadium that was built most likely required an EIS because it was being built in the swamps. You'd probably have to do several environmental studies such as a noise, fish, bats, etc. I've heard the budgets for EIS are 6 figures. Not cheap and usually the owner tries to change the project to avoid drafting the EIS.
Environmental Impact Statements are a result of the National Environmental Protection Act (NEPA) and are required whenever federal money is spent. Then there are a bunch of special cases (Categorical Exclusions) and situations where an Environmental Assessment is all that is necessary (no significant impacts). Unless the Giants Stadium was built with federal money, an EIS was most likely not required. However, the city of San Francisco would've required rigorous studies in the planning process, and lots of public participation, akin to that which happens during an EIS.
One difference between the two is that an Environmental Site Assessment is a "statement of condition" of the environmental aspects of a site. It is usually geared to those sites that either produce something that could have an effect on the localized environment or site that uses products that have an effect on the localized environment. It is an indicator of the potential of a given site to affect, through contamination, adjacent contiguous sites.
An Environmental Impact Statement is an assessment, typically for the influence of a large site or multiple site development on its neighbors or the region and incorporates things such as the impact of a development on the efficacy of the surrounding green areas, wildlife, quality of life, downstream runoff and other direct environmental impact.
"I've heard the budgets for EIS are 6 figures" - That sounds like a bargain. I've heard that the EIS for the Tappann Zee Bridge in NY is approaching NINE figures.
An EA or EIS is required when federal approval (not $) is required for construction of a project. The determination of whether an EA or EIS is needed is made by the Federal Agency (or the courts) in "lead" over the decision to approve a project. An EA is for relatively simple situations, and the Federal agency in charge can issue a FONSI at an EA level. If however, the project is larger, or more controversial, then an EIS may be required. Note that an EIS in itself doesn't stop a project, and the law just requires the Fereral agency to recognise the impacts before approving a project. On the other hand, the lawsuits that accompany accusations of incomplete or faulty EIS preparation can make progress really really slow.
