The amounts the IRS allows for housing and utility expenses has come under fire from TAXES.COM and tax practitioner groups nationwide. The IRS allowances do not reflect the current state of the housing market in many areas. Despite that problem, these amounts are what we have to use under the current rules.
The amount allowed is the smaller of your actual expenses for:
1) Rent or mortgage payment for the taxpayer's principal residence. Add the average monthly payment for the following expenses if they are not included in the rent or mortgage payment: property taxes, homeowner's or renter's insurance, parking, necessary maintenance and repair, homeowner dues, condominium fees and utilities. Utilities include gas, electricity, water, fuel oil, coal, bottled gas, trash and garbage collection, wood and other fuels, septic cleaning and telephone;
-or-
2) The amount found in the following chart from the IRS website (Collection Financial Standards, Housing) based upon: 1) The number of people in your household & 2) the county in which you live.

The first column is for one or two persons, the middle column is for a family of 3, and the column on the right is for a family of 4 or more.