Calculating Boiler size

Hi I'm about to buy a new combi condenser boiler, and have a few questions:

1. In the boiling sizing tools available on the internet, for a semi-detached house should I list it as having 2 length-ways walls or 1? (it has 2 walls obviously, but 1 is exposed, and the other is attached to adjacent house).

2. I have large loft bedroom with 2 single 1 meter radiators. Any idea how much this will add to the total kW??

3. Say the answer to the tool comes out as 26. How much leeway do I have in choosing a boiler size? Should I go for a 25 or a 30?

Many thanks in advance,
Garcia.

The bigger the boiler the better the flow rate on hot water, most (all) combi boilers modulate to heating demand, so output size (kw) is not now such an important issue (on the c/h side).
Anyone want to argue the toss?

Thanks baxpoti.

So presumably something like a 25kw will do for a 4 bed semi with 1 bathroom?

if the boiler was 'heat only' (not a combi) then you would have to work out the heating load (including 2kw for hot water cylinder), but with a combi just go for the highest kw you can afford as this will give better hot water flow rates. (i've got a 30kw/105000BTU combi that only serves 7 small radiators, which only require 12kw max).