Does real estate I own outside Ontario count toward the Estate Administration Tax calculation?

Generally, no — Ontario's Estate Administration Tax is calculated on the value of the estate for which an Ontario certificate of appointment is sought, and real estate located outside Ontario typically isn't included in that Ontario probate value, because it isn't property an Ontario court's certificate actually governs. Land in another province or country usually needs its own local process, sometimes called ancillary or resealed probate, to transfer, separate from the Ontario proceeding.

This doesn't mean out-of-province real estate escapes taxes and process entirely, it can trigger its own probate-equivalent tax or fee under that other jurisdiction's rules, on top of whatever Ontario's Estate Administration Tax captures for the rest of the estate. It also doesn't affect capital gains tax at death, which applies federally to real estate anywhere, regardless of which province or country it's located in and independent of the Ontario probate calculation.

Because the rules for transferring and taxing out-of-province property vary significantly by jurisdiction, and because coordinating an Ontario estate with a separate process elsewhere adds real complexity, an estate trustee dealing with property outside Ontario should get advice both in Ontario and in the other jurisdiction before assuming either the tax treatment or the process will mirror what applies here.