Hello Alayan,
First, want to check 1.1.2.2 of Division A to see what building classifications fall under Part 3 jurisdiction. Anything that is not listed here, we would generally call a Part 9 building.
Next, you want to check Section 1.4 of Division A for the definition of building area. This tells us exactly how to measure the building area. Important note here is that floor area and building area are similar, but different and are used for different things in our codes.
Building area is the greatest horizontal area between exterior walls or between exterior walls and the centerline of firewalls, so in your example an overhanging portion of the building only counts towards the building area if it has exterior walls. Decks would not.
Coffee shop occupancy classification (letter type) would be based on the total occupant load (#of people). Going off memory here because I'm not in Ontario (this is an OBC only thing), but I believe if it is not more than 30 people, you can classify it as an E - mercantile, otherwise it is an A2 - Assembly
Retail is E - mercantile
Rooming house would be C - residential
Once we have the individual occupancies, we can classify the occupancy of the building. You will imagine a building of number of storeys your building for each of your occupancies. Look at 3.2.2.4 to 3.2.2.8 for some guidance here especially if you have small occupancies that might not need to be considered (less than 10% of the floor area of the storey it is located on). now we look at 3.2.2.20 to 3.2.2.82 for your 2 to 3 options based on if the coffee shop is an A2 or not.
The way the titles are laid out is the <occupancy><number of stories><sprinklered or not> ex. 3.2.2.59. Group E, up to 3 stories.
In the above example we have a group E that is permitted to be constructed up to 3 stories in height and does not require sprinklered (buildings requiring sprinklers simply state "sprinklered" at the end).This looks like it will fit for the E classification we found earlier, but it might not. This specific classification has a maximum building area based on the number of streets (streets is a defined term) and the number of storeys (we could use this classification for a 1 or two storey building as well). So, we have one of the classifications to compare.
Once you other ones and what we are looking for is the fire separation requirements. we need to compare what the required fire separations are for each building and the most restrictive case will dictate the classification of our building. For instance, in our example: floors, combustible mezzanines, combustible load bearing walls columns and arches must all have a fire resistance rating of 45 minutes. If your other classification requires an hour or more, or requires a roof fire resistance rating as well as the other ratings, then this will be the "building classification"
When a building official asks you what your building classification is, we want to know which 3.2.2 section you designed the building to.
Hope this helps and feel free to ask any other questions you have.