The crucial measurement is top of stairs height . You must have 2m ( as mentioned above sometime less is allowed (1.9) .
As far as regs are concerned you can crawl around in your loft as long as you comply to this .
Now clever loft convertors ( as long as you are aware what you are getting ) may pinch height with the use of landings or even large velux windows .
Bearing in mind you are probably not 2 m tall you can sometimes get a reasonable loft by doing this .

Now ! A couple of other options are to either lower ceilings ( usually in older homes or a procedure I've carried out over fifty times , Raise the roof ! Usually only possible in detached homes but we do this on homes that are brand new to hundreds of years old . By doing this this all of a sudden gives you a lot more space at a great cost per sq ft in less then 4 weeks .
Thanks Paul woodcock

check with building control and the conditions of your plans 2.1 or 2.3

Generally you will need around 2.1 or 2.2m of headroom for the majority of the room to make it worth while.You would need 2m clearance at the top of the stairs to conform to regs although in certain circumstances building control will reduce this but only marginally.You will have to get someone to take a look as without knowing what size floor joists,roof alterations your job requires it would be difficult to say exactly how much room you would have at the finish.Not all lofts can be done.

It all depends on how the floor is constructed, if you have a load bearing wall in or near center of your house the size of the joists will be 150mm high and 2.4m is needed to get 2m headheight, if no loadbearing wall and the span is 5m then you will need 225mm joists and 2.5 is needed, if the span is more than 5m then a spanbreaker steel will be needed, this will sit above your existing ceiling joists and is normally a 200x200 uc so2.7 will be needed. These heights are minimun to generaly get 2m, the more height you have the better the conversion will be.

Hi,
It's 2m on and at the top of the stairs/landing(build regs). There is no min for the height once you walk into your loft/room/wc(build regs). I wouldn't go lower then 2m if you can? But its upto you,,, it's your loft
Thanks Russell
MTR LOFTS

You will need to check when obtaining planning permission but height I believe is 2.3 metres and minimum space is 2.1 metres.