Depending on how the layout is changing, ensuring that no load-bearing walls are being moved, and also if plumbing fixtures are being added (which it sounds like there will be) you may need an Architect.

Here are the things we keep an eye out for when it comes to renovations:  (1) major components go first - roof, electrical, central air, plumbing, (2) plan, plan, plan... 

@Samvel Gevorgyan I use all hours plumbing.

The garage already has plumbing, it's own yard area and enough height for a loft. 

This means a rental unit that:o Is secure, with doors and windows that can be lockedo Has working plumbing, hot water, heating, and at least one smoke detectoro Has functioning electricity without unsafe wiring or outlets.Your landlord must tell you about any known lead-based paint or hazards.Your landlord must keep any common areas and elevators in clean, usable condition.You have the right to basic privacy in your home.

After living in an 1896-built Victorian for the last 12 years, I can give you these recomendations:Update the kitchen and bathrooms, redo the wiring and plumbing, keep the original woodwork, arches, assuming they are still in good condition. 

I can do 90% of remodeling/repairs/plumbing/electrical work myself unless I’m away on business.

Quality work will go a long way.To give an example, an investor I know just bought a house that needed a layout restructure (basically redoing the layout of the house to make it more functional and attractive), installing central HVAC, building out a washer/dryer room, plumbing (new bathroom and kitchen location | rough in + top out), full electrical rewire (after full demo to studs), new sheetrock, paint, flooring, kitchen/bath finish out, and exterior restoration.

I would also tell him that you would like to have an annual property inspection report, which should include a plumbing, electrical and HVAC inspection.  

But then after that, you would need to buy a single-family home and work with a realtor and ask them to help you look for properties that have the potential to be converted to a duplex - which can be challenging, because their inherent structures themselves might not be suited for what you're trying to do (especially if you're trying to add a bathroom or separate heating, or plumbing units.