This is a bit tricky.
The original Ontario Heritage Act (OHA) prohibited Individual (Part IV) designations in a (Part V) district. As far as I can tell, the reason for this was that property owners should not be subject to two designation regimes — both of which were new in 1975 — applying to the same property. But since Part IV designations took off much sooner than Part Vs, which were slow to be embraced by municipalities, the result over the years was that municipalities wanting an Heritage Conservation District (HCD) were often faced with the choice of repealing existing individual designations to clear the way for a district, or of having “holes” in the district where district plans and controls would not apply.
The case that really highlighted this issue concerned Port Dalhousie. In 2001 St. Catharines opted to repeal three Part IVs in Port Dalhousie in preparation for HCD designation. There were formal objections to this, I think by people who were understandably confused by what was going on, and this triggered the Conservation Review Board process. The Board supported the repeals in 2002, but as a result the HCD was delayed and wasn’t approved until 2004.
To address the issue the OHA was amended in 2002 — as one of the “efficiency” changes made that year — to allow Part IV designations in districts. (See subsection 41(2) of the OHA.) The amendment meant that Part IV controls and process would apply to such properties; but it was also expected that, where there was a district plan (not required for HCDs at the time and having the status of a guideline), it would be used to help guide review of alteration/demolition applications for the Part IVs too.
Meanwhile the province was preparing final proposals for the comprehensive amendments to the OHA which became law in 2005.
With changes coming to district designation, especially new requirements for a strong district plan, the policy question about the relationship between Part IV and Part V designations demanded a more sophisticated response than in 1975 and 2002. Rules were set out for which Part applies. (See subsections 41(2.1) - (2.4).)
These rules are not well understood in some places. The policy intention here is to ensure a more uniform and holistic approach, with the new, improved district plans and controls applying to all properties in the district, while at the same time recognizing that municipalities might still wish to designate individual properties in the district under Part IV, for reasons such as recognition, eligibility for Heritage Property Tax Relief and other incentives or for controls on interior heritage features.
Essentially, the law now says that, where you have a Part IV in a Part V, and the district has a plan adopted post-2005, the Part V controls and process apply with respect to alteration, demolition, etc. and the property is subject to the district plan. The exception is where there is a designation of interior features; for alterations to those features the Part IV process apples.
But if you have an “old”, pre-2005 HCD and plan, then Part IV applies (i.e. the 2002 rules) — until, that is, you update your plan.
Capiche?