Funny you should say that...!

There seems to be quite a trend going on for this type of thing about now! There is more demand for the type of flat such as you describe due to single person households, divorce rates and FTBs unable to afford a 'whole house' - so they buy half a house!
For a FTB these flats are a more appealing prospect than a part-share in a 'shared ownership' house, in my opinion.
Have you considered selling the freehold to the eventual purchasers too?
As for getting the plans... if there is a precedent for this type of development in your street, could you view any developments that are for sale currently? You will then get an idea of the potential of the rooms and how the space could be used, what works and what doesn't.
You could also make friends with some of the locals who may show you round their newly converted flats.
My advice is, don't play around with the layout too much, don't divide rooms unnecessarily (unless they are particularly large!). Large, airy spaces will sell the flats better than a series of cramped rooms.
My suggestion for the ground floor:
Block up the wall to the dining room (the middle room). The living room door (at the front) then becomes the 'front door' to this flat. Partition it in such a way that the front room becomes a bedroom.
Put a doorway between the bedroom and the middle room. The middle room then becomes the living room.
Access to the kitchen is through the living room. Put units along the window wall of the kitchen only. This might leave room for a small table and chairs on the opposite wall, or at least a breakfast bar (a good selling point in a small flat).
Access to the bathroom is through the kitchen. Place the bathroom door in the far corner of the kitchen wall, not the middle. This allows you to put a cooker against the bathroom wall, with a sink under the window.
First floor:
If you are lucky enough to have a house with two windows at the front, make the most of it by partitioning the room to give you a small bathroom at the front with a window.
Keep the chimney breast wall as a feature in the front bedroom/sitting room (the purchaser will decide how to use this space as it will be roughly equal in size to the middle room). This leaves you free to use the third (back) bedroom for the kitchen.
If there is only one window at the front, you could put a run of kitchen units along the wall opposite to the chimney breast and have one large sitting room/kitchen. It can be quite effective if you have the space. You can then use the third bedroom as a bathroom.
Alternatively, you could divide bedroom three in half across its length. Put the kitchen at the back (making the most of the width of the room and overlooking the garden) and the bathroom at the side. If you can fit a small window in the bathroom, so much the better.
I would market the flats complete with fitted stainless steel cookers, stainless steel sinks, neutral carpets, magnolia walls and NOTHING ELSE! Your purchaser will think, "Oooh, I could just move straight in!"

Good luck!

DD