Your posts about looking at homes, dream homes, your husband's picky list, your own....don't bode well. If you do a huge addition and still end up house-hunting in a few years (two years? Maybe, from what you write and your close, close following of the market), you will end up having sunk a lot of money into an addition and then moving anyway. It's fine to do an addition, even one that is much larger than your area usually sees, as long as you plan to stay there for years and years to come. That is the part I would put into boldface italics with capitals if I could: IF you plan to stay there for years and years to come. If you do an addition, it needs to be what you and he want to LIVE in until the kids are done with high school, maybe with college, or don't do it. If you are doing it with the idea of "how does this affect the house's potential future sale in a few years" -- you are not committed to staying in place. You have done a ton of house hunting and you seem extremely invested in the idea of a house whether it's the one you have, only bigger, or a new dream home. I would worry that the intense focus on the housing market, how it will change, etc. would end up making you dissatisfied whatever happens.....And your picky husband will be JUST as picky over an addition/renovations as he has been about houses you've seen while house-hunting.
Again, additions are great and people here do them -- they even do ones that, as another person here put it, are "overbuilt for the neighborhood" and won't make back what was paid for them, if the house were sold. But if they do that they are committing to stay and building what they want to live in for a long time to come. Can you and your husband make a firm commitment like that, or are you both going to end up always a bit dissatisfied, always eyeing the market, looking "just for fun" at houses, doing a lot of "what ifs" and still feeling you didn't get what you wanted.
Not the answer you wanted, regarding dollars and cents and loans, but something to consider well before you talk to anyone about a loan.