concrete floor/screed question

I need to raise the floor level of an area about 2.5 by 1.5 metres which is a covered area between the back door and the outside toilet and store which have the same floor height as the kitchen
The base is slabs on hardcore which have been in place for 60+ years and are flat and level
The area has been enclosed for years with wooden frames and doors and windows which I have removed
The infill will be 125 mm total
My plan is 100mm concrete on top of dpm laid on the slabs
construct new frames from 75 x 50 studding on top of this and enclose
Finish with 25 mm of screed to get finished level
from what I have read the screeding wont be easy but it is only a small area and dead flat is not necessary
I am looking at 4.5 sharp sand to 1 cement and a fairly dry mix ( it has been some 25 years since I last did any screed so any tips appreciated)
Does this sound ok or should I reduce the concrete thickness and increase the screed thickness?
Regards
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TMC wrote:

Little bit thin for screed, though I have seen worse and you'd probbaly be OK for such a small area - but is it subject to freezing?
An SBR modified screed is what I used when I needed only an inch thick - that can go as low as 10mm if you follow all the process.
--
Tim Watts

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not subject to freezing roof is 100mm concrete, stud walls will have 75mm insulation
Will be at least frost protection heating to start with for outside toilet and washing machine in the store area which will be open to the new enclosed area
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