If you're serious about doing all that, break down and spend $2,500 or so and have plans drawn up along with hard estimates of the cost to build it. This is such a generic question with so little info that not even a contractor or architect could tell you off the top of their head.

Here's a link to cost vs value for 2015.
http://www.remodeling.hw.net/cost-vs-value/2015/

The other question to ask is 'will this add value'. Loft conversions don't add as much value as bedroom additions as they aren't 'really' part of the home. No addition gets 100% return of course.