Mayor Walter E. Washington has signed legislation that will assure District of Columbia homeowners an automatic property tax credit of at least $109 in the 1979 tax year, which begins next July 1.

The mayor had previously signed emergency legislation providing a similar credit for a smaller number of property owners in the current (1978) tax year.

The 1978 legislation applies only to owner-occupied, single-family dwelling units. It produces the $109 credit by exempting the first $6,000 of the homes's assessed valuation from taxation.

The amount of the tax credit next year court vary if the tax rate is changed. For next year the legislation is broadered to include the owner-occupied unit in apartment buildings of five or fewer units and for housing cooperatives.

The mayor also signed legislation setting the real estate tax rate at $1.38 per $100 assessed valuation and the personal property tax rate at $2.82 per $100 during the 1979 tax year. The two levies are expected to yield $207.6 million.

Also signed was a bill updating the definition of historic motor vehicles, and exempting them from periodic safety inspection except at the time they are first registered. Such vehicles cannot be used for general transportation.

Present law, enacted in 1959, defines a historic vehicle as one produced no later than 1929. The new measure defines such a vehicle as one that is at least 25 years old, or is at least 15 years old if the make is no longer being manufactured.

Another measured signed on a permanent basis permits the closing of an alley in the Georgetown block bounded by 29th, 30th, K. and M Streets NW, to permit the construction of a development that will include a plaza, a seven-story office and apartment building, a five-story hotel, and a three-story townhouse.

The measure previously had been enacted on a temporary basis. The mayor's signing of the permanent bill without requiring the developer to pay for the value of the alley was protested by the Georgetown Advisory Neighborhood Commission.

All the bills must lay over in Congress for 60 legislative days before becoming law. In the meantime, the two chambers of Congress have the right to veto the actions, a step Congress never has taken.