Basement is a huge plus in any house. You could use it as a man cave or kids’ playroom, in-law suite or office zone. Even if you don’t have a separate entrance you can make one and have a nice unit that can bring secondary income. 

A proper renovation will give you a warm and cozy apartment with all necessary amenities. With some designer’s tricks you could add extra space and light. Before starting that major renovation make a list of things you want to accomplish. You need to check waterproofing of your basement to keep it dry and mold free. Another important thing is insulation that keeps the temperature comfortable in the living space. Hire a pest control company if needed. Those steps would help you to get rid of the musty smell that basements usually have. All these steps are important for any project in your basement. 

Small basements could fit a lot of shelves for your stuff, laundry room and extra freezers. Basements without separate entrances could be easily converted into a work out  room if you are handy. Also you can make a space that you could rent out for small events such as local master-classes. Of course you need a powder room and kitchenette to offer enough comfort.

Basements with separate entrances could bring some income and help you pay your mortgage. Those projects are more expensive and take longer since you need to get all permits, complete complicated work and get through all steps of renovation.  

However once it's done you’ll add huge value to your property.