I own a building with a shop below and apartments above. What policy?
Not a residential landlord policy on its own. Mixed-use buildings need a commercial package or a mixed-use form that contemplates both occupancies. The proportion of each and what the commercial tenant does decide which one, and both are questions an underwriter will ask first.
Does it matter what the downstairs tenant does?
Considerably. A restaurant or a workshop changes the fire exposure for the whole building, including the residents above. Tenant mix is one of the main things carriers assess on a mixed-use property, and it affects both price and appetite.
Can I keep the residential part on a normal policy?
Generally not, because the policy covers the building rather than a floor of it. One building, one policy, written for what the building actually is. Splitting it is not usually an option.