Hi Ash,
Firstly thank you for your kind comments.
I'll start with the bad news - you've been mis-informed about the lack of requirement for planning permission. Planning law no longer just relates to physical structures, it has been twisted in recent years to regulate density and a number of other issues that have become problems due to defects in other areas of the law.
If you intend doing this completely legally then you'll also need to go bald and have a 50/50 chance of going bankrupt in making the place comply with building regs, but if you've read my earlier post you'll already know that.
The informal conversion I suggested, whilst being illegal, is not exactly a heinous crime. It's down to your own individual conscience and how good a return on investment you will get as to how much fireproofing etc you install.
The principle is this - you convert the place very quietly and simply into two flats. Tell the council tax people what your doing but nobody else. Then rent the flats out on seperate ASTs and bite your nails for 10 years. After that time has expired and you haven't had a court order then you have two legally seperate dwellings.
A couple of experience-based hints: don't rent to HB tenants, the housing officer will make merry hell for you. If you quietly live below the radar, don't pi55 off the neighbours, make sure the council tax is paid, and stay out of the way of the housing department you stand a good chance of getting away with it.
The potential downsides are:
a) tenants dying in a fire if you make a hash of the design and/or fire safety precautions
b) housing kicking off, in which case you'll probably have to restore it to a single dwelling. They'll do this either by an enforcement notice, or the slightly snidey route of putting an HMO license on the place restricting it to one occupant. This can ruin the saleability of a building as it actually legally precludes a family or even a married couple living there. Both these routes are fairly unlikely, however, as they are far too busy targetting studio flats with 10 albanians living in them.
c) planning enforcement, pretty much the same as above
You need to balance the risks here against the potential savings, and allow for some or all of the above consequences to happen. If you do it right, your chances are good, but for gods sake take the safety of your tenants seriously.
I hope this helps clarify the issue somewhat.
"The power of accurate observation is commonly called cynicism by those who have not got it"