Residential and investment top-up mortgage on same property?

If somebody has maxxed out their mortgage capacity to buy the family home, what are their options if they'd like to develop part of the family home? Specifically, if part of the family home can, with further capital investment, be converted into residential property that could earn €3000-€4000 per month in rent, how could they go about financing the additional €200,000 cost?

If it's via a property investment mortgage, would the part of the family home that would be converted into rent-earning accommodation need to be converted into an investment property in order to secure finance? What is the smartest way of financing this, and what sort of professionals could I speak with who would know about this sort of financing (and the best way to deal with the tax, legal, and planning issues)? Thanks.