This is a frequent question, there are no regulations but guidance and it falls down to personal preference.
2.1m is seen as a comfortable minimum, but we have gone as low as 2m, with 1.75m into dormers. One point to bear in mind is the resale value; this may be compromised if you adopt a low conversion.
In order to optimise the design we suggest the devil is in the detail and will require careful consideration from the outset.
We have just launched a new design service for our clients. We would be pleased to provide you a quote for feasibility study to assess viability before committing to major project works.
Jeremy Wedge MIstrutE CEng on behalf of Loft Conversion UK