It wasn't that long ago when modern homes emerged two rooms together to maximise space. Lounge- diners or kitchen-diners where common place. How, ever today the trend is to have separate rooms for each particular function.
In order to achieve this look you don't have to start building walls inside your home, instead you can use room dividers to achieve the same effect.
If you find that you want to create a particular area into a more private one, then you can hold of a divider which is free standing, and position it where you like to have that private area, but make sure it does not go across any windows. Many conversions often have windows at each end of the building so you will need to think exactly where you place your divider.
Colour and design are critical to the overall success of the effect you want to have. Keep the overall theme the same, use coordinated wholesale fabric, wall coverings and floor coverings. With your windows, again keep them coordinated, this is best done by having all your windows hung with cheap Roman blinds or ready made curtains in the same colour.