The Three wall proposal..

Right, there are three possible plans which I've been calculating,
Bearing in mind that the boiler room will be above our new bathroom, it will require a new concrete pad built up there, and the walls will have to be able to take the strain of the beams, blockwork, concrete, walls and water tanks.

Option one:

Green is 150mm Ytong blockwork, yellow 100mm and red 50mm. 
Largest floor area, I'll know for sure that the walls can take the weight (2N/mm²) and everything will be nice and straight, with maximum space available, no idiotic thick walls, only requiring one lintel per wall instead of two or three.

The downside:
Each bearing wall will require its own footings, I have no idea what the footings are like for the existing wall, how deep they are or what they are made of, if the worst comes to the worst I'll need to make new footings completely, 1M deep, 30CM wide and however long the wall is, with reinforcing inside it, to build the new blockwork up onto.
All the other non-supporting walls just get 30cm to build the wall on, dependant on what is there already (I won't know until its too late, and half the rooms are demolished!) with any luck I'll uncover existing concrete footings!

Cost: €3900 (if footings are 1M deep, considerably less if they are not.)
17M³ of concrete (if 1M deep)

Option 2

Knocking down the walls between Granny's room and our living room, they are non-supporting and easily removed. I'd replace both walls which gives a slightly larger kitchen and bathroom, as well as adequate support for the boiler platform above.

Same plan as above, except now I'm making mini-footings for the non-bearing blockwork to sit on, on each side of the bearing wall, and also full footings for the area where the boiler platform will be.

Cost: €3300,- 

14M³ of concrete (if 1M deep)
Option 3

In this version the original walls remain completely, but there is also the least amount of space, the bathroom for example becomes 2.1x1.9M and kitchen 1.8x1.9M Some of the walls become 55cm+ thick.

These "foundations" are very easy, simply making a ledge all around the existing walls for the blockwork to sit on. All I'm unsure about is the load imposed above the kitchen/bathroom on the existing walls.

Cost: €3600,-
11M³ of concrete.

Boiler slab:

 Here's an image of the Ytong block and beam I'm proposing to use, you simply place the beams on a solid surface, and fill them in with the blocks, then put a covering of concrete over the top, or reinforced concrete, and then continue building the walls upwards.


Roof Beams: 

And also an image of how the roof beams are situated, and which walls they are resting on.
All that rests on those beams is the weight of the roof, and since I already need to take down a fair amount of the walls to make doorways, and enlarging a window and creating an opening to the kitchen, I don't really see any harm in replacing the supporting walls, with more modern materials which take up way less space, and will require way less heating.

The fireplace in the livingroom is another point, the chimney will more than likely require to be replaced, so that whole corner of the room needs to be removed as well, and new foundations laid for it, if you ask me, it makes way more sense to replace the lot, it's "only" €300,- more (If the footings are 1M deep)  and 7M³ of concrete (if 1M deep) than simply building a new wall around the existing wall.