Pre-qualification is a quick, informal estimate based on self-reported numbers — good for early budgeting, not much else. Pre-approval involves a lender actually verifying your income, credit, and assets, and results in a conditional commitment letter. In the Bay Area, sellers and listing agents expect a pre-approval letter (not just pre-qualification) before they'll take your offer seriously, so I recommend getting this done before you start touring.

You can put as little as 3.5% of the purchase price as a down payment with certain types of financing. However 20% or higher is more attractive particularly if there is a lot of competition for the property. (Tip: check in with your lender and agent to discuss which makes the best sense for your scenario!)

There are four items you'll need to have to make a strong offer on a home in the Livermore Tri-Valley: 

1) Pre-approval letter from a reputable financial institution (if financing the purchase).

2) Proof of funds for your downpayment and closing costs - showing that you have the funds available. 

3) Buyer Representation Agreement - this document confirms your agency relationship, responsibilities, and agency compensation.

4) Residential Purchase Agreement - commonly referred to as "an offer". The residentail purchase agreement outlines all of the terms of your purchase including price, contingencies, escrow period, items included and excluded from the sale, who pays for what - and more!