Does severing a lot from farmland trigger a property tax reassessment in Ontario?

Generally, yes. Severing a parcel from a working farm creates new, separate legal parcels, and MPAC typically reviews and reassesses the resulting properties, since a severance changes the boundaries, uses, and sometimes the eligibility of each new parcel for the farm property class. A severed piece that is no longer actively farmed, or that becomes a standalone rural residential lot rather than part of the working farm, can lose the reduced farm tax rate on that portion and move to the full residential rate, even if the remaining farmland keeps its farm classification.

The remaining, still-farmed parcel generally keeps the farm property class as long as it continues to meet the usual eligibility requirements after the severance, but this isn't automatic either — the reassessment process looks at each resulting parcel on its own facts. Because the tax consequences depend on how the land is actually used after the split, and on the specific size and configuration of what's severed, anyone planning a farmland severance should get advice on the expected tax outcome before finalizing it, and should not assume the new parcels will simply carry forward the same classification and tax treatment as the original, unsevered property.