A lot of cowboybuilding is going on lately with too thin inner walls . The following is correct ;
 102.5mm inner wall has to have a 75mm cavitywall . A 100mm cavitywall has to have a 4.7 inch inner wall.      This is for domestic house size dwellings.                            Concrete upper floors have to be on at least 5 inches wide .inner wall.
Brick and stone walls or smooth concrete walls must NOT be "rendered". 
Thin o.p.c. mortar skim "render" is only for o.p.c. concrete breeze blocks (the mottled variety ones ONLY) to a maximum of 2mm to 3mm only (no more) ; Any thicker and it would be DETRIMENTAL. 

Walls must be the same brick all the way up . or the same concrete all the way up . or the same stone all the way up from d.p.c. 
Ask the true CITY & GUILDS N.V.Q BRICKWORK for advice