I purchased a house in 2017 that has a legal ground level suite (muni. counts it as a separate residence). Can I claim a portion of my purchase cost as a capital cost?

If you are renting out a portion of your Principal Residence, any expenses and CCA are prorated according to the percentage of personal and rental used. 

There are many factors to take into consideration when claiming Capital Cost Allowance on a rental property, and claiming it is "optional". You may want to have a look at the following links for more information. 

https://turbotax.intuit.ca/tips/dos-and-donts-cca-for-rental-property-explained-6377

https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/t4036/rental-income...