Hi Jenny,
If the lender only ordered a 'drive-by' appraisal they probably are not too worried about it not appraising (at least that's the only time I see drive-by appraisals requested). The loan to value ratio needs to be 80% or less to avoid paying PMI, but otherwise it depends on what type of loan you're getting. This is a question for your lender.
Make it a great day,
Seth T. J. Miller
Broker- My Real Estate Pro West Covina
(626)967-8686
DRE#01840251
PS- you can follow my on twitter @BrokerSeth - Sun Jul 12, 2009