Can you give us a bit more info on how you live, state, family size, etc?

Variation's of these narrow dwelling's have been done many time's before. The issue is that many are inner city suburb's, with tight building control's, heritage overlay's, etc.. so the second storey can be a tricky one to advise on. Lower level not too bad.

I would remove the existing WC/Laundry/bath completely and have them located where the existing living is, unless the living has a fireplace as this is a feature to keep. If there is a great fireplace to keep, have laundry/bath tucked between existing living and kitchen.

The rear addition would be a kitchen/meals/living similar to your third drawing. You probably don't need to increase floor size but layout it out better.

Note: work on stair position first then work from that. It can be tweaked here and there but they're such a lumpy thing that they will determine the usability and layout of the whole dwelling.

If you leave them where they are they will continually be in the way and compromise the design thought process. I don't believe the WC/laundry/etc should have what is possibly the best view, give the best view's to living space's where majority of our time is spent.

Not expensive in whole scheme of thing's. If you can afford to add a second level and renovate the existing then relocating the plumbing/etc is neither here nor there. The existing plumbing may well need replacing anyway. Wouldn't surprise me either that the flooring/wall's/roof have issue's as well.

If they're newly renovated to high standard then they may be worthwhile considering however.

as already mentioned this style of property is subject to special heritage, height, size and set back constraints that must be clarified before a floor plan should be considered but, working within the existing building, my suggestions have provision for a new bathroom and stairs in the old living area with a study nook and storage under the stairs, there are two kitchen and stair positions with new cabinetry and a moveable island to allow space for a dining table, the old bathroom and sunroom are replaced with a new living area...Layout of upstairs will be determined by set back and height which decides the available space but the central staircase should allow a variety of options for upstairs btw there was a similar design Dilemma by HOUZZER Bailan czch last November and you should check out that post for relevant ideas and information

Your area can be a bit tricky to get thing's passed painlessly so engaging a local designer is a necessity. The council just require a lot of report's on various thing's, probably due to the significance of the area, small site's with limited access and space, noise, high density living, etc.. Doesn't need to be an architect as there's a lot of building designers and draftsman in your area as well. Have a chat with them before deciding the best way to go.

I like both of oklouise's concept. I would just locate laundry under stairs or in place of the study nook as I like to have garden views from living space's.

I like the living area at the end of the room. Move the kitchen along toward the living area to make a narrow laundry room along the back of the bathroom and in/under the upper stairs. It would be also cheaper at this point for plumbing etc.

the kitchen should have a return breakfast bar and put the table opposite the kitchen area.

An open office nook could be put into the corner opposite the laundry room.

Hi bigreader, that's a fantastic point, although I'll struggle with space.

i don't think there's enough room allowed for the stairs or for allowing space to get on and off the stairs or movement of furniture up the stairs, bathrooms and laundry are very small with difficult access and there's too much space for hallways..by swopping the stairs there's enough room to keep your new kitchen, dining, living, decks, balcony and rooftop but there now enough space for bigreader's separate toilet, generous space for easier access stairs with walk in pantry downstairs and wardrobe upstairs, full sized bathroom, laundry and ensuite

Thanks Oklouise, a real challenge is whether I have enough space to move things around. The only issue I have with your suggested floor plan is the location of the stairs, as we prefer the hallway uninterrupted. Plus we also prefer to have an indoor garden upstairs. Perhaps we could rethink about the location of laundry and increase the space for stairs.

here's a straight through downstairs hallway but not knowing what you need for the indoor garden i've concentrated on minimum spaces for bathrooms, laundry, stairwells and essential walking spaces with some leftover for the upstairs gardens