No, most summerhouses do not require planning permission. They're classed as a 'permitted development' – just like a shed or greenhouse. Unless they have a pent roof over three-meters high or an apex roof over four meters high.
With the right wall, floor and ceiling insulation, your summer house should be warm enough to use all year round. Consider investing in double-glazed windows, carpet and wall hangings. You could even add a stove or electricity for radiators.
Build your summer house on solid concrete or a paving slab base. Seal the roof to avoid leaks. Install gutters and clear regularly. Frequently apply timber treatment. Inspect seals around the windows and doors. Ventilate often to avoid damp patches.
Concrete or paving slab bases are the best because they're solid and permanent. However, you could also use composite decking, which is low maintenance and hard-wearing. Or timber decking, which offers air circulation but needs regular maintenance.
Proper ventilation will keep the air inside your summer house fresh and stop dampness from forming. The simplest way to do this is by installing an electric or solar fan and ventilation grilles. You can also open the doors and windows every few days.