Loft conversion height promises - who do you believe?

[Frogponder]

I wonder if anyone can shed light on why companies give such different assessments of how high rooms would be if we convert our loft. I was just advised by Simply Lofts that our 2.3m high ridge beam will definitely give a loft room of 2.1m height. Another company said we'd only get 2m height, a third said definitely only 1.9m if it is done properly (ie only by them !) and we should lower all the ceilings to make the conversion worth doing.

Our existing ceilings are high enough to lower 10cm, but as we're in a first floor flat, doing this would mean putting nearly everything in storage, moving out for a few weeks, more mess, stress and expense. The height issue is critical, but how can we decide what to do when we get quotes varying by 20cm??

Any mumsnet wisdom much appreciated!

Maybe to do with the amount of insulation and/or the depth of the joists (a larger number of shallower joists rather than fewer deeper joists). Losing 10cm for the floor and 10cm for the roof sounds about right tbh.

Less than 2m head height over the stairs is illegal.

2.3 meter clearance now will be fine though. You’ll achieve over 2m.

We were told we didn’t have enough head height by a couple of people but the loft conversion people managed it without lowering ceilings. They did something with the ridge tile which made it just within regs and it was all inspected and signed off. So I would say the simply lofts people know what they’re able to achieve!

Thanks @BananaPie and @Karwomannghia -
that's reassuring!