Between commissions, appraisals, points, application fees, and so on, you might pay eight to ten percent of the cost of your current home. It's also common for people to do some remodeling to the home they move into. Throw in moving expenses, too. And there's higher real estate taxes and maintenance costs. Add all that to the difference between the price of your current home and the one you want to buy.
Financing costs is another factor.
Mortgage rates could be less than the cost of borrowing to remodel. Compare that total to the cost of remodeling or adding on. Compare square footage. One way to compare apples to apples is to calculate all costs on the basis of square footage. Of course, this may work in your favor if you move from a hot housing market to a cooler market.