Layout of the house

We have been struggling (and still are) to settle on a layout for the house that meets our requirements and seems doable, practically and budget wise.

The main stumbling points have been fitting in a mud room (given how close the door is to the existing stairs) and working out where the bathrooms/toilets/sinks can be to allow the waste to flow in to the septic tank. This second point complicated by the fact that we actually have two septic tanks at the moment, one is not really a proper official one. The guy we bought it from added it just before the sale completed after the survey which covers septic tanks had been done. I think he was trying to be helpful as the sink was just emptying outside, but it is a worry to us as it drains away under the house. It only has the kitchen sink water going into it so it could be worse.
It is also difficult to get the flow between rooms on one side of the house and the other to work well, I think this is because the house is quite long and thin.

We are still not sure if we can have a solution where the kitchen sink drains out the back of the house as it does now, but we have worked out some plans where this and whether there is enough clearance under the stairs for a corridor are the main questions we can see at the moment. They involve getting rid of the old stairs and replacing them with new, this is a bit of a shame as the stairs are original. But they are also rather woodworm eaten and would need a lot of work anyway.
Something else we are thinking about is whether to have underfloor heating upstairs - we plan on having it downstairs but would like wood floors upstairs and are not sure if under floor heating is a good solution or not.

We are thinking of getting a local architect to draw them up properly as plans so we can make sure we haven't missed anything as well as getting the plumbing and electrics included on them. I haven't found any magic program on the internet allowing us to do this ourselves yet, plus they have experience spotting things that would not work.

The idea of the bed that seems to be splatted on top of the stairs on the first floor view is that it is on a raised platform above the stairs... just an idea...

First floor