The Internal Revenue Service is after Spiro T. Agnew for back taxes again, in part this time because it contends the former Vice President didn't spend as much to house, food and clothe his family as the average American family did.

Agnew's attorney filed a petition in U.S. Tax Court yesterday disputing the IRS claim that he owes $13,966 in additional taxes and penalties for 1973, the year he was forced to resign from office after pleading no contest to a single charge of tax evasion.

The current case is not criminal, but the IRS is seeking a 5 per cent "negligence" penalty against Agnew.

The IRS determined that the former Vice President earned $26,717 more in income in 1973 than he reported on his federal tax return. It said he should have reported as income $11,370 in home improvements by the General Services Administration, $4,398 for personal use of government aircraft, $924 in food delivered by a grocery chain executive, and $5,554 in personal and family expenditures.

The tax collection agency also disallowed business entertainment and home office expenses deductions totaling $3,600 and the exemption Agnew claimed for supporting his aunt.

In Agnew's petition his attorney, Myron J. Mintz, labeled as "harassment" the IRS finding that his client failed to report the $5,554 in income used for household expenditures.

The IRS computation compares Agnew's personal and household expenses with Bureau of Labor Statistics figures on what an urban family of three would have spent.

In effect, the IRS seems to be saying that the Agnews must have spent at least as much as the average family of three, though it could find no financial records to support even those average expenses and therefore Agnew must have paid some household expenses out of unreported income.

Mintz said in an interview yesterday that the procedure of measuring a particular taxpayer against a class of average taxpayers was highly unusual and irregular."

He said the facts would show that Agnew did not have a lot of personal expenses during 1973 because of his position as Vice President.

The case calls to mind an IRS audit of former President Ford, which determined that while he was in Congress he got by on about $5 a week in incidental expenses because of the [WORD ILLEGIBLE] of his office.

"Why can't they use a rapier instead of a bludgeon," Mintz said of the IRS method of computing the additional $5,554 in personal income.

In the petition, Mintz also contested most of the IRS' other claims. He said the $11,370 in GSA improvements on Agnew's Kenwood, Md., home - which included a brick wall, and a remodeled staircase and plumbing - were done "for the convenience of the U.S. government" and were installed for security reasons.

The use of government aircraft was required in carrying out his duties as Vice President, the petition said. The IRS did not show how it reached the $4,398 figure the agency alleges he owes for personal use of the planes.

The $924 in food, which was delivered by Joseph Rash, a vice president of Food Fair Stores, Inc., was "a gift made from detached and disinterested generosity," the petition said.

After his resignation on Oct. 10, 1973, Agnew settled a civil fraud case with the IRS for unreported income for the years 1967-72. During that time, according to a Justice Department accounting read at his tax evasion plea, the onetime governor of Maryland had accepted more than $87,000 in cash payments from Baltimore engineers.

The current IRS case was delivered to Agnew on April 15, the day the 3-year statue of limitations in civil cases ran out.