I wonder about how many of these actually be built on foot of this. Those people with large gardens who would like one for their adult kids or an au pair probably already have one that is occupied as such already. Those who might want to make a quick buck will potentially be put off by the soon to be very inflated cost of installing one and potential bottleneck of supply - even if you can get a ready made one installed, it still needs a raft, plumbing, electrician sign off etc. Those trades people are pretty hard to get these days. You can apparently move and sell some of them but again, how does this work in practice, you need a crane to lift, some way of transporting, a crane to put in it's new place and the same issue with raft, connections etc.
What will probably happen is those who already have one who might have been put off by not wanting to have hassle with neighbours reporting them for using them without planning might reconsider their value if they can legally use them as a rental.
Also what will the knock on impact be on converted garages, granny flats etc? If i had one of these that is not exempt from planning for rental, I can't really see the difference and I would be demanding a review of this too.
Full disclosure we do actually have one, use it daily as a home office and it is very well insulated and feels like a proper room - the quality can be very good in them and as a renter quite private even if in a garden of a residence versus say a large apartment building or a house share. Alas no plumbing otherwise I'd consider moving in myself and renting the house out!