Instrument 41,
My recommendation is, hands down, Sketchup. We hired a designer earlier and it was not a good experience... all we got was what they wanted and little consideration was given to what we wanted. Next, we worked with an architect and no builder would touch those plans (seriously, five different builders said that a house could not be built from those plans). We spent a considerable amount of money with the designer and architect and had nothing to show for this other than a lot of lost time and a considerable depletion of our bank account. I used Sketchup to design our home which was then was built - we moved into it in November 2017.
Since you are using part of your original structure take a look at this -
https://help.sketchup.com/cs/article/3000115. It gives some good instructions on how to draw your home from a photograph using Sketchup - this is a great way to play around and look at various designs. You can, of course, go around and measure everything but that is not necessary at this stage (the photograph method is quicker - I used this approach earlier and took four pictures of out home from the four corners and applied Sketchup textures). You can also purchase an inexpensive "Sketchup For Dummies" book from Ebay for cheap! -
https://www.ebay.com/itm/Google-SketchUp-For-Dummies-For-Dummies-Computer-Tech-ExLibrary/273195696187?epid=57573328&hash=item3f9bbb803b:g:XBwAAOSw4A5YnbAX. Be aware that Sketchup licensing may have changed - I always used the free version but had difficulty the last time I tried loading it on my computer (I went and searched for Sketchup 2017 and it worked wonderfully). Maybe after the two of you arrive at a creation that you like THEN you can talk to an architect...