My husband and I would like to add on to our house

We’re in debt to Sallie Mae (29,000)and the mortgage (104,000). We have a healthy
emergency fund (33,000.) Our housing expenses (mortgage, property taxes, home owner’s insurance,and home maintenance sinking
fund(153/mo) all add up to 18% of our take home pay.
Initially, my dh did not want to pay the student loan with our EF, butnwanted to use the EF money (20,000 of it) and a loan (60,000) to pay
for an addition (80,000.) He thought we’d just pay Sallie Mae forever. Ahhhh….
I don’t want to borrow money for an addition, and would like to wait till we saved up the money for the addition. I may have successfully convinced him that it’s a good idea. But…He now believes we could pay more toward the student loan and pay it off in 2yrs. In that 2yrs we’d save as much as possible to pay for the addition. Then after being debt free except the mortgage… take out a loan for 22,000 (80,000 minus how much we’ve saved in the past 2yrs(38,000) and 20,000 in our EF). Our housing expenses would still be under the 25% of our take home. That would include a first mortgage, second mortgage, higher property taxes, higher home owners insurance and higher home maintenance sinking fund (210/mo). He doesn’t want to wait the extra year to save up all the money for the addition, so this is his way of meeting me half way here. I’m open to his ideas, but don’t want to go ahead with a bad idea. The option to pay off student loan and buy a bigger house that is 25% or less of our take home crossed our mind, but it’s actually a lot cheaper (70,000 savings) to do an addition in our situation/location. Plus, we’d have to sell our house first and most of the houses in our neighborhood are selling at reduced prices. And, I’m an architect and would rather design the addition for the way we live. Also, we are estimating the higher property taxes, insurance and home maintenance sinking funds… Does anyone have any advice on getting more exact numbers before we do the addition? I’ve thought about calling the county tax collector, insurance co, but I’m not sure how much to put in a home maintenance sinking fund. I just thought it’d be about 1% of the value of the house divided by 12 months. Oh, and should all these costs be included in the 25% of our take home? I’d rather be conservative with my calculations, and not be surprised later with being house poor.