In which case, if you don't mind about having to live that way forever, I do know of places which are concealed and have been there for many years. Locals know about them but aren't bothered as the occupants cause no problems and look after the bits of forest or whatever that they inhabit. One day the tree fellers will come, and they will have to move on presumably.
If your idea was to get the Certificate of Lawful Use, and you are in England (I don't know the rules for the other countries), then landroverroy gave you the salient piece of information; anything which could be considered fraudulent could be used as a reason to not grant the CLU. But you would need to talk to a planning consultant in your area to know whether your scheme would be considered fraudulent; it's open to interpretation. Getting planning permission for some stables and living in them for 4 years then applying for a CLU would be pretty unequivocally fraudulent. But building and living in a well-concealed dwelling, possibly not so much, depending on your local planners.
However, as landroverroy also told you, it's 10 years for a caravan, so if you are planning to do this you might want to consider doing something which is not a caravan - ie., is not something which can be driven / towed away - in which case it's 4 years.
It is not illegal to build or live in a structure; it is only illegal to continue to do so in contravention of a Planning Enforcement Notice. So if no-one ever reports it, and you meet the elapsed time requirements, then you can apply for the CLU. (If you think about it, if it hasn't caused any issues resulting in someone reporting it for many years, then is it really a problem to anyone for it to be there?)
There are lots of hoops : it has to be a recognised eligible dwelling, which to most planners means it has to have running water, sewerage and power, a bedroom, bathroom and kitchen; you have to be able to show continuous occupancy as your main / only home for the relevant period (and holiday letting does not count, it has to be being used as a home continuously); etc. It is for the planners to prove that you have
not, but you do need some evidence that you
have. Take pictures and record key dates, like when services were connected, when you moved in, and so on.
Again, a local planning consultant would know what your local planners look for. As a start, you could have a read of applications and decisions made on previous applications in your area (search "Certificate of Lawful Use" in your local online planning register.)
Lots of good info on the
Chapter 7 website and in Simon Fairlie's book (details on the Chapter 7 website.)