Reasonable estimates on "stand alone" construction are difficult enough at a distance. However, the great unknown for me is how it should be linked to the existing.
Ground floor is probably obvious enough, even first floor is guessable from experience, but the battle is won or lost in the conversion of the existing roofspace. There are so many variables to take into account. So many questions to ask, all requiring inspection.
Standard practice is to get a few quotes, but even this would be difficult without some sort of drawings to work from.
This really does need a structural appraisal before any kind of meaningful price could be offered.
It all sounds perfectly possible, even quite straightforward, but I'm afraid you're asking too much without some kind of drawn scheme.