Unfortunately, if an extension would make the middle of the house much darker, a conservatory is likely to have the same effect. You might be able to employ an architect who can come up with a way of rejigging your downstairs layout and including an extension to give you larger, separated rooms all with decent light (and absolutely no internal rooms). That's likely to be much more expensive than just tacking something on to the outside of your house though as it would mean removing load bearing walls etc.
Looking for a house is really depressing. So much of what is on the market is really dismal. We looked at houses up to £300k (which we could have afforded but didn't really want the large mortgage that would have gone with it) and found that made us even more disappointed by how bad the houses were for the asking price. We saw some truly awful things.
Our personal favourite was a house where the couple were divorcing (probably because of the house!) that had an unfinished bathroom (no ceiling, mismatched scraps of carpet on the floor, one of those green glass bowl (this kind of thing) that had been stuck into the top of a really old, cheap, white Formica unit) and the most bizarre extensions imaginable.
It was a 1930s semi and they'd left the tiny original kitchen (with lovely units that were probably from the 70s or early 80s) and tagged an extension on to the side. They'd incorporated 2/3s of the garage into this extension and then extended the garage to the side. The extension composed of a weird shortened, extended garage bit at the front with a garage door and a white uPVC door right next to it. On the back wall of the garage was a small window (on a completely internal wall) and a door. The door led into a very dark narrow corridor (no natural light) with some worktop along one side. Off the corridor was an entirely internal room, with the window looking out into the garage bit, that had strange dark brown wallpaper with a Japanese art style pattern on the walls and a freestanding bath plumbed in the middle of the room. Nothing else. DH described this as 'the room where they cut the bodies up', Round a corner the corridor continued with a door into the original kitchen, a small shower room, a tiny separate toilet and a back door. There was also a horrible 1980s conservatory on the back that was completely empty.
It was utterly bizarre (and if anyone is reading and thinks, 'that sounds like my house', do please explain how it came to be like that). Obviously the sellers weren't doing their own viewings.
We also saw a house on a hill with a basement extension that included a horrible, windowless room that looked like the sort of place people lock their kidnap victims up in horror films. Not a selling point!