Appraisers take into consideration "The Dirt" that the home is on. We do not value the house but the "Property". If this is a bank required appraisal, it is necessary as the bank wants to make sure that there is "Hurt Money" as a minimum in the amount not financed. So a 20/80 ratio means that the buyer has 20% money in if and only if the house appraises. Many states (I have been license in 7 but not Texas) actually will allow a buyer to void a contract if the house appraises for less than the transaction price unless the seller is willing to lower the price to the appraised value. An appraiser will look at condition in addition to pure "comps" but don't expect that baking cookies is going to influence the outcome. In many areas, banks will require a higher down payment if the property does not appraise for the transaction price, again to make sure that if they have to foreclose, they can discount the property, sell it quickly and get at least what they are owed out of it. Traditional banks do not want to be in the Real Estate Business with an inventory of houses., which is why they have aggressive REO departments working with experienced brokers.
One of the things that you may find on a bank ordered appraisal is a very conservative one. Say the transaction price is $500,000 which the appraiser will know, expect that if the house is worth at least that, that the appraiser will probably appraise at the traction price or only slightly over as to not stick their neck out. A lot changed on the appraisal field post the September 2008 mortgage melt down. If you want to get a better idea of the true market value of your property, hire an independent appraiser that has not bias to any interested party.
Bob