I can build a 40sqm extension to my house without needing planning permission and just following a few regulations. But as it must be attached to my property anyone staying in any room in the extension would be in my house.
Now, if they allowed me put a self contained modular home of 40sq m on my property with a few regulations (such as restrictions on who it can be rented to), I'd probably do that and rent it at a reasonable price to to either my brother in law or a student (via a college accommodation manager) or a Ukranian person.
And with so many large rear gardens in many cities, something like this really should be considered - obviously you could not have 40sqm extension + 40sqm modular home.
And as they would be moveable structures, finance would be easily arranged.
A few rules and regulations would be attached, but It would be a very quick way of getting more rooms added and the tax free recognition payment would make it financially attrractive