What size in sq. footage and how much? Was this a turnkey deal where a flat price paid for everything? How much will the value of the house increase? Is there a kitchen and bathroom? If no kitchen, then how much more? How long did it take to put yours in? Did they come out with cranes to do it? Does the siding match up? Does the window type and size match up? Did they put the stairwell to the second floor in a convenient place on your first floor so it looks nice? Did you have to spend anything for flooring and walls where the stairwell went in? Did they tear off and use a new roof or did they use a crane to pick up the old roof? Did they put this in and remove the old rafters without tearing up or dropping something through your first floor ceilings? Did they tie into existing HVAC and furnace or did they put a new one in for the second story? Were they able to tie into the existing plumbing without tearing up anything for drains and vents? Did they have to run a new electric panel or did they use a subpanel for the second floor and did they tear up anything on the first floor to run the wiring? What kind of basement or foundation do you have? Did they do this without tearing up your yard and landscaping? What did it cost to replace any lawn and landscaping?
You really have my interest. Please answer the above questions to help us make recommendations to others. Since you said and recommended that this is the only way to do it and you don't have to move out and that it was done in 7-10 days that you have already had this done and are and can speak from previous experience.