Tenants at a Northwest apartment building, tired of going without a security telephone answering service and air conditioning for the past several years, agreed to a substantial rent increase to pay for the services.

Their agreement is an example set by several landlords and tenant groups who are taking advantage of a little-used rent control measure called Voluntary 70 Percent Rent-Increase Agreements to negotiate for specific improvements in exchange for higher rents.

"The increase wasn't something that was forced down our throats. We wanted these services and knew they were going to cost," said Mildred Isler, one of the residents at 1431 Summerset Place NW.

Isler's landlord, Thomas O'Donnell, president of Waggaman-Brawner Realty Corp., said: "This is the new way of rent control that should have been done a long time ago."

Under the voluntary agreement, at least 70 percent of the tenants in an apartment building must sign a petition for a rent increase either to improve services or to provide for capital improvements, maintenance and repairs.

If the landlord initiates a voluntary agreement, a detailed rent-increase proposal must be presented to tenants, who have a minimum of 14 days to confer with other tenants and respond to the landlord. Similar rules apply to tenants if they initiate the agreement.

The rental agreements have been available to landlords and tenants since implementation of the District Housing Act in 1977. But tenant and landlord interest in them did not increase significantly until about three years ago, said James Hampton, acting housing administrator for the city's Rental Accommodations Office.

"It is a phenomenon in the housing industry. For the first time, the landlord and tenant are not adversaries. Through the 70 percent agreement, they can mutually work together to communicate and tailor-make their needs," Hampton said.

"Landlords and tenants are realizing that in the midst of high rents and a shortage of quality housing, the voluntary agreement is the best way to solve existing problems," O'Donnell said. Hampton said the 70 percent agreement can be processed quickly, usually within 30 days. Two other ways landlords can increase rents, through the financial hardship petition and automatic rent increases allowed by the city, usually take twice as long to process, Hampton said.

Under the 70 percent agreement, residents have the right not to pay the additional rent if landlords do not provide the services within a designated time, Hampton said.

So far, the Rental Accommodations Office has processed 145 voluntary agreements, which is slightly lower than last year at this time. For fiscal 1982, the city rental office processed more than 311 agreements involving 2,707 units with an average rental increase of 39 percent. Most of the buildings averaged nine apartment units or less.

The agreements have a 98 percent approval rate from the city rental office, he said. Hampton also has the authority to disapprove an agreement if either party is coerced into signing the petition, if the proposal is misrepresented or if the proposal does not comply with District housing codes.

Michael Scott, also a city rental office administrator, said the 70 percent agreement tends to have better results in smaller apartment buildings where tenants and landlords communicate frequently.

"It creates a new environment to educate tenants and landlords while changing hostile attitudes. The tenants come to realize that the landlords must have these increases if they are going to have a fair return on their buildings. Meanwhile, landlords hone in on the specific needs of the tenants. It is the new way," O'Donnell added.

But Don Slatten, executive director of Apartments and Office Building Associates in the District, disagrees. "The 70 percent agreements don't work too well. They are being practiced, but not a whole lot. The biggest problem is that it is very difficult to get 70 percent of the tenants in a building to do anything. It just doesn't fly financially," he said.

Another landlord, Byron Christenson of Daro Realty at 4301 Connecticut Ave. NW, said it is also difficult to get tenants' consent on such an agreement when they do not have confidence in the landlords.

"Most tenants truly believe the landlords are out to take them," he said. "Even if you show them the figures and explain what is going to be done, they still think you are fudging," he said.

Some tenants, however, see the voluntary agreements as an answer to nagging housing problems.

The agreement for Isler's building will not go into effect until October. Isler, like 24 of 28 other tenants, consented to a 30 percent rent increase over two years on her one-bedroom apartment in order to help pay the $40,000 cost to install the security system and air conditioning in the building.

"We are very, very happy about it. Last year we had asked for the services but couldn't seem to get anything done about it. This year, we felt we were going to get on top of it," she said.

Another resident, James Amick, said, "the agreement is fair. The landlord really worked with us."

Bill Pettis, the resident manager, added, "The residents felt very honored that we consulted them about what had to be done to the building. There were no threats. Everyone was congenial and responsive."