The first thing that pops to mind when people consider buying a tiny house is... Where can I put it? The garden is the obvious answer as they fall inside all the regulations needed to site a "Garden Building" in your garden as long as it is for personal use.
Apart from your garden or a friend’s garden, you may need planning permission however it is not impossible. All it takes is a few forms to fill out, the application fee of approximately £200 and an 8-week wait. A lot depends on the use of the land where it is to be situated. Is the land Green Belt, is it Agricultural Land or an Area of Outstanding Natural Beauty. The other thing to bare in mind is the use of the Tiny House. Is it to live in as a permanent dwelling, is for temporary leisure use or do you intend to rent it out as a small guest house/holiday home? All important factors that your local council will want to know before granting planning permission or agreeing that it would fall under permitted development.
The best advice I can offer (as all situations are different) is visit your local council, armed with a photo of a tiny house and an intended location and ask their advice. After all, councils are there to help!