Accessory dwelling units can be plain or fancy, large or small, but all designs need some sort of heating and air conditioning as well. Deciding on a heating system for the ADU isn't that easy if it's a freestanding unit. When it's connected to your house, you might be able to extend central heating ducts to the unit. But a freestanding unit needs a system of its own, and central heating might require more construction than you want to deal with for a small ADU. There are other options that you can have installed.
Space Heaters: Be Careful
You could provide a space heater, but be very careful about the type you provide. Look for one where the heating unit is raised up off the floor; if you go with an oil-filled model, ensure the unit is sealed so that you don't have to worry about refilling it or that the tenant will use it after the oil has been used up (in sealed units, the supply does not change). Also, look for a model with an eco-type setting that reduces power use and lets the housing cool down to a reasonable warmth. The big disadvantage to the space heater is that it can be taken out, and you don't want to keep losing space heaters as tenancies turn over.
Wall Units: Efficient but Requires a Hole in the Wall
A hotel-style wall unit is a permanent solution, and while most people see these in buildings that need them for air conditioning, many models have both heating and air conditioning. Installation, once the hole in the wall is ready, is fairly simple, although it is a good idea to have a heating installation company do this. A disadvantage to these units is that the hole has to match the unit, so if you replace the unit, you have to find something that's the same size as the hole through the wall. And then there's the mere fact that you need a hole created in the wall.
There are also electric and gas wall heaters that rest in the wall without having to stick out the other side, but these too require that a hole be created. That can be a long process if you've gone through all that construction to build the ADU already.
These heaters also cover only one room, in most cases. For a studio ADU, that's fine, but for a 1 or 2-bedroom ADU, it may not be ideal.
Ductless Mini-Splits: Efficient for Multiple Rooms, With Little Wall Work
One solution that could be great for ADUs is the ductless mini-split. These systems still require punching a hole through the wall, but this time, it's a small one meant to accommodate a hose. The hose connects an interior unit with an exterior unit, and you can connect more than one interior unit to the part that's outside. So you can install these mini splits in multiple rooms but have them all run off the same motor.
For more information about a home heat system installation, contact a local contractor.