We bought our 1200 s.f. 1915 house in 1980, about the size of your original you want to keep. TheHusbandHalf built a garage in 1987, and we had the intent of putting an addition connecting the 2, if finances allowed.
Then we added an addition, about the shape and placement of what you're thinking, plus an added sun room (not a 3 season room) on the back of the addition. We hired a guy to build the shell, liked what he was doing, so had him put new windows in the old part too, and had him and roof the new, to match the old, and side the whole thing.
While he was building the addition shell, we lived in the old, with our 3 kids, 7,5,4. We used the front door for 8 mos or so. When it was time to open up the back wall of the old, we hung some heavy clear plastic to help preserve heat, and in fact left it up until the furnace for the new was working. (We have completely separate HVAC for the old and new now – comes in handy for times when one needs maintenance). We're in Ohio, and we hired the guy to do the job starting in January. Looking back, it was probably average temps, and average snow for that time of year.
THH knew enough to keep an eye on things, in fact with the proper equipment and helpers, he could have built it. He got the job he has now about a year earlier, so it just made sense to hire the guy. We did the rest.
The kids had fun drawing on the subfloor, and running around like crazy kids. I think the roller skates were used too.
I think our first concern was the heating and the insulation. The addition, like the old, had 2 x 4's for the exterior wall. THH did the same in the addition like he did in the old house. He added wood to each stud so it ended up we have 6" thick walls, meaning more insulation. (It was cheaper to do it that way than have him use 2 x 6's for the exterior walls.) Downstairs we added a new family room, sunroom, mudroom to the garage and a back porch, plus a closet under the stairway up to the master bed and bath upstairs, plus a walkway to the old upstairs. Our septic tank is the size for a 3 bedroom home, and that’s what we ended up with.(The old house had 2 bedrooms because we had made the downstairs 3rd into a half bath and laundry room.
Something I really like is the old stairs to the old upstairs is still as it was, and we have a new back stairs, which makes a continuous loop. There was an exterior upstairs door in the old house, we just kept the opening.
The fact that the walls were open, and we did the electricity, allowed us to make the outlets and switches on the outside walls, air tight.
We redid the old bathroom so the tub/shower for that, and the tub/shower to the new master bath were lifted up when there was an opening in the framing. They are one piece units, popular back then, and I still like them – they will stay as long as I do.
Where the old hooked to the new, if I remember right, there’s a wooden beam going across above, and the ends are supported by wood posts to reinforced old foundation. That way the whole of the old foundation was not being used for the whole addition. I remember when they dug the hole, he placed what I call pillars (probably the wrong word) and they support the floor. I remember he had a line of the along the old foundation, probably to carry most of the weight.
It was a big day, for me at least, when they delivered the 28’ steel beam, and put it in place, so we were able to have 28’ with no walls. Then the sunroom is attached to the back
In our case, it was cheaper because:
1.We hired the guy and he worked when work was slow, so I suspect his price was a little lower (not positive of this though) He was just a small time builder, with the reputation for good work.
2. When it was airtight, with heat, we worked on it as time allowed. I always say, “A house is not something we have, it’s something we do!)
We have always gotten permits whenever a permit was needed. About a year after the main building was finished, I was home alone and the county ??(not sure what her position is called) was at the back door to check on our addition. She kind of looked around where she was, did not come in past the door, and said it looked how they did it when they were young. It was obvious some things were not ‘finished.’ She said she was just going to write it down as finished, I suppose just for more tax money. To us, it was great so that we really could go as slow or as fast as we wanted. See, in the 2 counties to the west, especially the nearest, there’s a big problem if you don’t finish the work in a set amount of time. That county, I’ve heard, is much stricter. That is the county our builder was from, so they knew his work too. Our county is more relaxed, more ‘farm.’
Just my opinion, but if you think the additions that were put on are not of the quality, or design, that you want to live with, tear them off. If you don’t, there will always be ‘something’ that will bother you, and you’re just going to put too much work into the home to not get what you can live with Like I said, just my opinion, but that’s the way THH has always thought. Maybe that’s why we always get permits? We try to not cut important corners. One of the reasons we bought our house is because no one had come in and done any major work.