There'd be a lot of waste with that with pipes and electrical connections leading no where and at best lots of extra framing around the non existing doors that would be cut out to install the new section. There's also the issue of how to deal with the roof and cladding that would have to be sealed completely between the two sections against water and pests. It's doable but there's a lot of unavoidable work that'd have to be done to connect the two bits properly on top of whatever additional site preparation to extend the footprint of the foundation.

That work would be a big portion of the cost of adding on to a house much like additions are made now.