It is a real difficulty with this kind of arrangement. Our house is a strange set up because our living areas and kitchen are upstairs. We've blocked off part of the living area with a gate, so DD doesn't have access to any other room. I guess you need to be extra vigilant. Because I've had this arrangement since day one, I have always supervised her. It didn't really occur to me until I was at a friend's house the other day and she didn't seem concerned at all about where her child was because she knew her house was safe. I have to keep my eyes on Louisa at all times.
It must be frustrating for her to have such a small play area, but it's the only way we could arrange things. I make sure though that now that she's very mobile I take her with me into other rooms and outside so she gets a change of scenery.
My main worry, though, was the railing over the stairs. Now that she's climbing on everything, I've had to shift all furniture away from there so she can't use it to climb over. I hate to think about that! Their skills develop so quickly and the moment I discovered her standing on the coffee table was the day I realised I had to rearrange everything!
Don't forget the bottom of the stairs too. Kids will attempt to climb them and then injure themselves falling, so a gate here might be a good idea too. My gates are the hardwood ones from Babyco so they still look okay.
Although our kitchen is blocked off, other parents I know who have open plan areas just make sure that drawers and cupboards that contain dangerous items are kept closed with latches etc. Even thick rubber bands work. Keep a couple of bottom drawers for tupperware/plastics and allow them access to these to keep them amused. Even the saucepan drawer if you can stand the noise! Also fridge magnets are a big hit.
That's all I can think of for now. Hope this helps.
DD Louisa (One year old!)