Poorly designed and incomplete plans rushed through by engineers are a major factor in the delays that currently beset developers and local governments in dealing with proposed land development plans in Northern Virginia, a private study has concluded.

The backlog is causing developers to grow nervous that financing for their projects will collapse, robbing them of summer construction.

For years, developers have pointed the finger at county bureaucracy for delaying new projects. Now, for the first time, an independent study by the leading management consulting firm Booz, Allen & Hamilton Inc. of Bethesda says the development community is partly to blame for bottlenecks in the sites and subdivision plan approval process.

"Poorly engineered plans are significant underlying causes of delays," the report on the Prince William County land development review process states. Many of the engineers in Prince William County also work in Fairfax County.

Between 70 and 80 percent of final sites and subdivision plans submitted for review in Prince William county omit essential data required under county code, such as roads, drainage and size of water lines for fire-fighting, the consultant found after a five-month study.

However, some engineers are skeptical that bad plans are to blame for this backlog.

John Elkin, partner in one of Prince William County's leading engineering firms, Bengsten, Debell, Elkin and Titus, says bad plans "are rare in my experience." More often, staff and private engineers disagree over the interpretation of county codes "rather than anything substantive," Elkin said.

In Fairfax County, Claude G. Cooper, director of the Department of Environmental Management (DEM), describes a problem similar to that detailed in the report. He estimates that 50 percent of plans submitted to his department are wrong or are missing vital information. County Executive J. Hamilton Lambert stated in a memo to the board of supervisors April 8 that engineers trying to get a jump on the review process are submitting incomplete plans and then revising 80 to 90 percent of their plans after submission. This practice is a "major contributor to wasted staff review time," he said in the memo.

These findings indicate that just as the unprecedented construction boom in Northern Virginia has stretched county planners and building inspectors thin, so, too, the development community is feeling the pinch.

"They've got the same pressures that we have," said Martin E. Crahan, director of the Department of Development Administration (DDA), the agency in Prince William County responsible for plan reviews and building inspections. "They are pushing, and it comes out in incomplete plans."

"It may reflect their choice of never saying no to a client. The difference is we never have the choice to stop processing plans," Roger W. Snyder, planning director for Prince William County, added.

Fairfax county's deputy county executive for planning and development, Denton U. Kent, agrees that the inferior quality of engineering plans has "become more of a problem in the last year," prompting his county government to launch an internal audit of its plan review process.

The delays come when construction in Northern Virginia is hitting record highs. The total value of construction in Prince William County has increased 154 percent between 1981 and 1984 -- from $80.2 million to $202.4 million last year. It rose 27 percent between 1983 and 1984, and in 1985 the county's construction is projected to rise another 21 percent to $247 million.

The figures in Fairfax County are even more dramatic. Construction topped $1.1 billion in 1984, a 19.4 percent increase from the year before.

As a result, the Fairfax County staff currently is handling 376 major site plan and subdivision submissions. Booz, Allen estimates site plans are taking the county on average five months to approve for a first submission and up to 8 1/2 months for a third review.

Plan approval is a little better in Prince William. Booz, Allen found site plans take between 2 1/2 and 7 1/2 months to win approval.

But Elkin, with the Bengsten, Debell, Elkin and Titus engineering firm, contends incompetent county government staff members are the real stumbling block.

"Personnel in many cases do not have the experience or education to spend a lot of time dotting 'I's or crossing 'T's. As a result, the quality of comments we get back don't amount to a hill of beans," Elkin said.

The Booz, Allen study in Prince William gave some credence to this theory. High staff turnover -- especially at DDA, where the 1984 attrition rate was 20 percent among its 87 employes -- is robbing the county of qualified personnel, which may contribute to delays through backed up, inadequate reviews or insufficient guidance to private engineers, the report says.

But in general the Booz, Allen study exonerates county staff. Detailed technical reviews, far from being nitpicking affairs, are "consistent with the responsibilities of public agencies to protect health, welfare and safety" and are in order given the "historical pattern of major errors of omission and commission" in plans submitted, the report says. It does add, however, that there is room for improvement in the quality of reviews performed by staff.

Crahan acknowledges that a 74 percent increase in work load, a reduction in staffing from nine to six full-time positions and a high staff turnover depresses the quality of work.

To address these problems, he has asked the Prince William Board of County Supervisors to authorize 16 new positions in fiscal year 1986 for his department, a 15 percent pay raise for plan reviewers and building inspectors, and a 22 percent budget increase. Except for the pay raises, all this could be funded from building permits and plan fees, Crahan said.

The Fairfax County Board of Supervisors April 8 approved a 10-point program designed to speed up the review process. This includes limiting alterations to 20 percent of the project before the whole plan must be resubmitted. The changes also require fourth submissions of plans to be sent directly to the division chief or to Cooper and permit a "quiet time" of four hours a day when plan reviewers and engineers cannot be interrupted.

Booz, Allen & Hamilton is expected by the end of the month to complete its final recommendations on how Prince William County can speed up its development review process.

Other bottlenecks in Prince William that Booz, Allen identified were:

* A zoning ordinance that lacks flexibility, resulting in overuse of special use permits or protracted negotiations between developers and staff over suitable zoning categories.

* Duplication of work performed by staff in DDA and the planning office and lack of coordination between these offices, especially over proffers and waivers.

* Inadequate systems for dealing with high-priority projects, causing other plans to get bumped to the back of the line.

* Preliminary plans submitted with excessive detail, hampering the review process and showing that developers lack an understanding of the purpose of these plans.

* Planning commission schedules that create one-month delays by handling projects in eastern Prince William County separately from those in the western part of the county.