A summer house is usually seasonal
Traditional summer houses are typically light timber buildings intended for warmer-weather use, garden seating or storage. They can be lovely spaces, but they are not necessarily designed to hold a comfortable internal temperature through winter.
That makes them a good answer when the requirement is occasional seasonal use and budget is the priority.
A modern garden room is designed as usable interior space
A year-round garden room uses insulated construction, proper glazing and doors, an electrical installation and a more complete internal finish. Heating, data and lighting are normally part of the design conversation rather than later additions.
The goal is for the building to feel like a detached room, not simply a shelter in the garden.
The price difference reflects more than appearance
Structure, insulation, membranes, glazing, electrics, heating and internal finish all add material and labour compared with a basic summer house. If two products look similar online but sit at very different prices, compare the build-up and included services before assuming they are equivalent.
Choose based on how often you will use it
If you want somewhere for sunny afternoons, a summer house may be all you need. If the room must function as an office in January, a gym before sunrise or an entertainment room throughout the year, specify it as a true garden room from the start.