What is it?
A phase one environmental site assessment is a term set by Ontario’s Environmental Protection Act. A qualified person follows the regulations to judge how likely it is that contaminants have affected the land, or the water on, in or under it.
Ontario Regulation 153/04 describes two general objectives for a Phase I: a preliminary determination of the likelihood of contamination, and deciding whether a Phase II assessment is needed.
What does it look at?
- The site’s past uses (factory, repair shop, gas station, dry cleaner, storage of waste oil or chemicals).
- Nearby land uses, underground tanks and earlier environmental reports.
- A site visit and interviews by the professional.
- A conclusion: whether the chance of contamination looks low, or a deeper study (Phase II, which actually tests the soil and water) is needed.
What to watch out for
- A Phase I judges “likelihood” from records and a visit. Confirming real contamination may need a Phase II.
- The law does not require one for every purchase, but a lender may ask for it, and it matters if you plan to change the use. For the rules on a change of use, see “Record of Site Condition (RSC)”.
- An older report may need to be updated. Also check who ordered the report and who may rely on it.
What the law and official sources say
Quotes are copied word for word. The "In plain words" lines are our explanation, not an official translation.
“phase one environmental site assessment” means an assessment of property conducted in accordance with the regulations by or under the supervision of a qualified person to determine the likelihood that one or more contaminants have affected any land or water on, in or under the property;In plain wordsAn assessment by a qualified person, following the regulations, of how likely it is that contaminants have affected the land or water.
Source: ontario.ca/laws/statute/90e19 · checked Nov 2, 2026
The general objectives of a phase one environmental site assessment are to do the following: 1. To develop a preliminary determination of the likelihood that one or more contaminants have affected any land or water on, in or under the phase one property. 2. To determine the need for a phase two environmental site assessment.In plain wordsThe aims of a Phase I are a preliminary judgment of the likelihood of contamination and deciding whether a Phase II is needed.
Source: ontario.ca/laws/regulation/040153 · checked Nov 2, 2026
This Standard establishes the principles and practices that are applicable to a Phase I ESA. It is intended to provide a consistent framework and minimum requirements for conducting Phase I ESAs that can accommodate broader regulatory and liability requirements, as well as address pertinent site-specific requirements.In plain wordsThis standard aims to give a consistent framework of principles, practices and minimum requirements for a Phase I assessment. The full standard is sold, and this box shows the text of its introduction page.
Source: store.csastandards.org/store/product/Z768-01/ · checked Nov 2, 2026
Common questions
Who does a Phase I?
Ontario’s Environmental Protection Act says it must be done by, or under the supervision of, a “qualified person”. An environmental consultant usually does it. Cost and time differ by site, so ask for a quote.
What if the Phase I result is bad?
If it finds contamination is likely, a Phase II can test for it. Options include a lower price, negotiating with a lawyer how clean-up responsibility and cost are split in the contract, or not buying.
Guides that use this term
Sources checked
- Environmental Protection Act (Ontario) · checked Nov 2, 2026
- O. Reg. 153/04 · checked Nov 2, 2026
- CSA Z768-01 introduction · checked Nov 2, 2026