The lender will not approve you on a "what if" scenario. That would be bad business on their part. If you get the preapproval now, as the time gets closer and if the house is not complete yet, you could have the lender run a new scenario using her new income. At that point, if you're approved, the only thing it would do is change your DTI though so it's not a big deal there. Usually if you're worried about income, it would be wanting to get the raise first so you can afford a larger home. If you signed the contract already, it doesn't matter.
If you're trying to calculate DTI and how much home you can afford, here's a simple way. Take your yearly gross income and divide by 12, so for you that's $8,166. If you get a conventional loan, your current debts including a new mortgage payment, taxes, insurance, HOA fees, etc cannot exceed 43% of your income. For you that means you cannot go over $3,511/month. If you get an FHA loan, your current debts, including new mortgage payment, taxes, insurance, HOA fees, etc cannot exceed 56.99% if your income. For you that means you cannot go over $4,653/month. It's important to note that there are lenders who have overlays, which means they have additional rules on top of the minimum allowance is.
As far as your student loans, if you go the FHA route, it is required by law to use 1% of your student loan balance when calculating your DTI. Even if the payment on your credit report shows a lower amount. It is only conventional loans that will take your current payment into account.
Also, if you're buying new construction, depending on the builder, you will not need the money for the full down payment the same day you sign the contract. Some builders will initiate a payment plan especially if the home isn't built yet. For example, the day we signed our contract, the builder wanted a $1,000 payment. We were then put on a monthly payment plan from March-June to pay around half of the downpayment. The other half is due at closing.
