Justice McKinnon’s decision in Osman v. Heath sets out nicely the principles of adverse possession. Perhaps surprisingly to those who don’t practice in the area, these are limitations principles determined by the Real Property Limitations Act. Here are the relevant paragraphs:
 The cases on adverse possession are legion and each case turns on its own set of particular facts. In Ontario, adverse possession claims are governed by sections 4, 13, and 15 of the Real Property Limitations Act, R.S.O. 1990, c. L.15, which establishes a ten-year limitation period in which a dispossessed owner must bring an action to recover possession once a right to possession has accrued. By section 15, when a person has not attempted to recover the land within ten years after the right to bring an action or make entry or distress accrued, the right and title of the owner of the land is extinguished. A person claiming a possessory title as against the legal owner must establish the following:
- Actual possession for the statutory period;
- That such possession was with the intention of excluding the true owner; and
- That the true owner’s possession was effectively excluded for the statutory period: Pflug v. Collins, 1951 CanLII 80 (ON SC),  O.R. 519 (Ont. H.C.); Marotta v. Creative Investments Ltd. (2008), 69 R.P.R. (4th) 44 (Ont. S.C.); Keefer v. Arillotta (1976), 1976 CanLII 571 (ON CA), 13 O.R. (2d) 680 (C.A.).
 The claimant must meet each of these three criteria and time will begin to run against the owner from the last date when all three are satisfied: Masidon Investments Ltd. v. Ham (1984), 1984 CanLII 1877 (ON CA), 45 O.R. (2d) 563 (C.A.).
 Marotta is a particularly helpful decision; it sets out in detail the applicable law, and I shall briefly follow the analysis employed in that decision.
 The claimant must establish actual possession for the ten-year period and the acts of possession must be open, notorious, constant, continuous, adverse and exclusive of the right of the true owner. In Teis v. Ancaster (Town) (1997),1997 CanLII 1688 (ON CA), 35 O.R. (3d) 216 (C.A.), at paras. 14, 16, Laskin J.A. explained the requirement of open and notorious possession in these words:
First, open possession shows that the claimant is using the property as an owner might. Second, open possession puts the true owner on notice that the statutory period had begun to run. Because the doctrine of adverse possession is based on the true owner’s failure to take action within the limitation period, time should not run unless the delay can fairly be held against the owner….
The element of adversity means that the claimant is in possession without the permission of the owner. If the claimant acknowledges the right of the true owner then the possession is not adverse.
 Further, the “inconsistent use” test does not apply to cases of honest unilateral mistake: Cunningham v. Zebarth Estate (1998), 71 O.T.C. 317 (Ont. Gen. Div.). The “inconsistent use” test does not apply in circumstances in which the person in possession operates under the honestly held belief that he or she is the rightful owner of the property or in cases where the legal owner and person in possession operate under a mutual mistake as to title or boundaries. In such cases, an inference may be drawn that the occupier is in possession of the land with the intention of excluding all others including the legal owners.
Actual exclusion of the true owners
 The final part of the test for possessory title requires that the true owner be excluded from possession. In analyzing this subject, the conduct of the owners in relation to the land is considered.
 As I have stated, the true owners had effectively abandoned the large shed certainly when the business was moved to another location in Kemptville, and probably during the 1980s. When the Doucettes acquired the Residential Property they closed off all entrances to the large shed on the side of the Commercial Property. It was effectively sealed off from access by the true owner. The sealing off was accomplished openly and notoriously. The entire building was raised and leveled, concrete was poured, and work was carried out on the exterior. Photos show Mr. Doucette on a ladder performing renovations to the exterior of the large shed. The true owners had been excluded from the large shed since at least 1990.