This depends on what the summerhouse is being used for and how often it's being used. Although interior walls and floors can be left untreated, treating them can help to protect the wood, make cleaning far easier and add some colour and style to the interior of your summer house.
Hard Wax Oils such as Osmo Polyx Oil and Fiddes Hard Wax Oil will help to protect the natural wood of the floor from dust, dirt and grime whilst making it easier to clean. Walls and ceilings can also be painted, varnished or oiled if desired. It's worth noting that clear oils and varnishes on pine timbers will draw out the natural colour and grain of the wood. In some cases this can produce a fairly strong orange or yellow colouration to the wood. For this reason we always recommend trialling a test area before proceeding with the project.