Bathroom plaster

Greetings all,
We've recently moved into a 1900 terrace, with a backyard extension comprising kitchen downstairs and bathroom upstairs. We want to pull out the bathroom fixtures, tiles, etc and replace with new flooring, wall tiles, suite, etc.
The thing is, the bottom half of the walls seem to have double the depth of plaster compared to the top half, ie the walls from the floor up to about 5 foot stick out about an inch further than the wall above. The previous owner had attached wooden beading around the lip, but SWMBO insists that we have flat walls top to bottom, ready for tiling.
The plaster is sturdy stuff, but I've taken about a foot square off, back to the brick, with the intention of plasterboarding to the level of the top half and tiling. There is a definite break in the type of plaster in the bottom half and the top half of the wall, and where it protrudes there are two layers of plaster compared to one above. Before I continue, I have a few questions...
Would this have been done when the extension was built, or at a later date? Why was it done? I don't want to take the whole lot back to the brick and have the walls collapse... Perhaps I should point out that this double-layering is only on the exterior walls - the wall between bathroom and bedroom, and between our bathroom and the neighbour's is flat from top to bottom, if that makes things clearer.
Any advice appreciated...
Tom
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GI Joe wrote:

I've seen this in a 1930's house we had some time ago. It was the kitchen, and the lower 4-5 ft of walls were rendered about 0.75 inch and a white tile applied. This brought the wall to the right thickness for a quarter round tile of about 1.25 inch radius capping that was applied at the junction of the upper and lower wall. My wife had a very unpleasant few days knocking this all off with a bolster (before cheap power tools).
R.
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GI Joe wrote:

The 'extension' you speak of is probably (highly likely) part of the original house and this is how tiles were affixed back then - they rendered the wall with a strongish mix of sand and cement and placed the ties on while it was still wet, the main plaster that they used was sand and lime, which has no adhesive qualitites at all. The reason why it is thicker is because sand and cement doesn't remain workable for any length of time if applied thinly. If you have removed all the tiles, try a level up the wall and see how much gap you've got, you may be able to dry line it with 9.5mm plasterboard and tile directly onto that. Why the other walls are not done in the same way I don't know, unless they weren't tiled at the same time as the exterior ones .
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Phil L wrote:

If you are tiling any old method of arriving at a level wall is good. I use bits of scrap timber, ply, MDF - anything to get a relatively level surface.. Any smaller gaps are filled with bonding plaster. Or sand cement render, or polyfilla or car body filer or in fact tile cement..whatever is to hand and surplus.
Only if its to be painted would I bother to skim it level, because with tiles, if you rip them off they need a skim afterwards anyway..leave it for the guy who wants to wallpaper it after you've sold it ;-)
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Thanks for the comments. So is the general consensus that I should rip it back to the brick, then fill it (be it with platerboard, aquapanel, plaster, whatever) up to the level of the wall above?
The alternative would be to plasterboard the top half out to the level of the bottom. However the plan was to tile the bottom half and paint the top (except around the bath/shower where it'll be tiled to the ceiling), and just do a quick skim on the area to be painted..
Tom.
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