Consider your septic system
Determine if your septic system is large enough to handle the extra load that year-round (or winter-weekend) living will put on it. This is especially true if you plan to install a washer-dryer, dishwasher, or extra bathroom. If not, you may have to put up with having it pumped more often, or replace it with a larger system, which may contravene local bylaws.
Heating and cooling considerations
Many cottages do just fine with electric baseboard heating for cool summer nights, but it’s a very inefficient – not to mention expensive – heating method in winter. If you have access to your town’s natural gas supply lines, you might consider adding a gas furnace, or installing a self-contained, high-efficiency oil furnace (modern-day ones are much more efficient than the octopus boilers of yore). In both cases, though, you’ll incur not just the cost of installing the furnace, but ductwork as well. A less drastic option is to install a wood burning stove or a fireplace in the main room (and perhaps another in a bedroom, if you have the room and the budget).
Even if the walls are well insulated, most heat loss and drafts occur around windows and doors. Single-paned windows, or even older-generation double-paned windows, should be replaced, and the frames insulated and sealed tightly. Consider replacing an old wooden door with a newer one with foam-core insulation, and/or beef up the weatherstripping and seals in the doorframe as well.
Lastly, consider hiring a snowplough service to keep the access road clear for you, especially if the cottage is on a private road. If you only plan to go there on weekends, have the snow-removal company keep the walkways and path to the front door clear as well; they can also monitor the snow load on the roof for you, and keep it from accumulating to excessive levels.