In theory no but the extended piece will crack through time (At the joint) and possibly drop a bit unless well dowelled into the existing floor. Suggest you speak to someone experienced about this and the type of mix to use.
[QUOTE 5309951, member: 45"]I have a friendly engineer who said it would be fine, and the previous step was shoddy and lasted nearly 50 years. I'm not too fussed about it dropping or cracking as it will be beneath a solid floor and wouldn't be a hard fix. I'm more concerned about the permanence of concrete coming right to the edge of the plaster.[/QUOTE]
Some expansion joint material possibly between the concrete and the plaster.
Some one has got there levels wrong ! (being a joiner/builder) You will be creating a step by bringing the edge to the front of the internal wall.
In your shoes i would take up some of the conservatory floor (100mm strip or take the edge off the existing conservatory slab) to form a graduate slope which would be less of a trip hazard and look less of a mistake by having two different floor levels.
[QUOTE 5310347, member: 45"]The step is unavoidable. The ground to the rear of the house slopes up away from the house, so this is the lowest we could get the conservatory floor without it going below ground level at the other end. And it's a concrete slab so there's no floor to slope.
The step will be fine. We tried to get rid of it but it wasn't possible, and it was there before so it's not an issue.[/QUOTE]
]The step is unavoidable .... But you have bridged the two damp course levels This should of been avoided in the first instance.
If that was my problem, I'd do the concrete, and finish so I could then put a hardwood threshold.
Finish with skirting and plaster to paint finishing.
A nice step has beauty and also a nice way to show the step up,down for any visitors etc.
Get the hardwood first so you can make the concrete step to the right height.
[QUOTE 5310397, member: 45"]I like that idea. I'm not sure how good I am at levels though and working it so that the solid floor ends up flush with the hardwood.[/QUOTE]
As long as you leave enough room for the wood you'll be ok as you can bed that on mortar or anything like gripfill ect.
You can set it level with spacers and screw it down then fill the screw holes with a complimentary wood filler so it blends in then varnish or wood oil etc to show off the woods beauty.
Worth it for a nice finish.
Concrete and a tile over option is ok but wood is king