Heatloss questions


Hi
Currently working out radiatior sizes for my house and have a couple of questions that hopefully somoene will be able to help on.
1. For heatloss through the ground floor, what is the temperature below assumed to be. I have a solid concrete floor which I assume has just earth below. Do I assume it is the same as the outside air temperature?
2. My lounge is open plan with the stairs to the first floor at one end, I do not intend to put a radiator on the landing (as it is quite small) so should I take into account the area of the stairs and landing and add to the heatloss I have for the lounge for the radiator size?. The radiator will be at the opposite end of the room to the stairs. A neighbour's system I have had a look at has only one radiator in the room and nothing on the landing, but it's size is smaller than I get for the calculation I have done for the one for my Lounge without taking into account the stairs - I'm confused to say the least.
Thanks in advance for any answers as I am stumped on these as I can't find any advice anywhere!
Cheers
Al
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