One of the first things, you need to find out, is if the house structure, will support a second floor addition. A lot of houses, will not support one.

There are many companies that do custom modular design. i dont know if you are familiar with the area i live in but it would conpletely unaffordable for us to move. We also live close to where we work, so moving is a no go.

Get a structural engineer or the custom modular company in for a consult.

We're nearby (MoCo) and recently did an addition to our small house. We think it was originally a small Cape or ranch and had two prior additions - one to expand the upstairs for a full second story (3br plus 1 hall bath) and a second addition to add onto the back. Those were done by previous owners in the 1980s, and the house needed a lot of updating along with more space. So we extended the second story over the back addition to make a primary suite for a total of 4br/3ba. We gutted much of the existing house and also added a porch off the back.

Ours was not modular, so I'm not sure how much this helps but it was $$$ and basically was a year from when we signed the contract to when we will finish the punch list (this week.) For 6 months of the bulk of the work, we were in a rental house. We talked to a bunch of local design/build firms but ultimately went with an architect and a separate contractor. Just to give you a ballpark, our bids -exclusive of fixtures, appliances, lighting, plumbing and tile - ranged from $310k to $475k (the porch was separate, $60k.) The addition was 500sf but we also replaced all windows and doors, added a fireplace, removed a wall, etc. And yes, like you, it was still a lot cheaper than moving. I hear from our contractor and architect that they have more work than they can possibly handle for the foreseeable future. Hope this helps in your decisionmaking.

Wondering if you decided to go ahead with the modular addition....we are in NoVa also and considering going modular