Site Specific Design Options

Adopting BIM to replace CAD is more than just learning this new tool. The tool is intended to do a slightly different job, and so it may not fit into your hands in just the same way.

In residential projects with multiple buildings, whether single or multi-family, one of the expediencies forced by CAD is that every instance and variation of every building is not fully documented. There is a ripple effect throughout the drawings. Changes must be followed through. And this is where the weakness of CAD becomes too obvious to ignore. It is extremely inefficient when it comes to the minor adjustments and fine tuning of the drawings. And so we do not do it. Because of CAD.

The logic is that since we are not responsible for the vertical placement of buildings on a site – that is the civil engineer’s task – we cannot possibly be asked to keep up with such changes. Instead we frequently refer to someone else's drawings, and it's up to the builder to put it all together in the field.

In the Playa Vista project, the documentation for all six buildings on the site is generated from a single model. The efficiency of using groups with Design Options is described in the previous post.

Building sites are never flat. In residential building, the threshold of the garage is always at least six inches below the finish floor level. The minimum standard is a four inch step and two inches of slope (one percent). 


In single family, the step-down may be increased to adapt the building to a sloping site. With larger buildings, in addition to the garage step-down, sometimes the entire floor slab must be offset, or even the entire building. Although these decisions are not within our scope, we are required to depict this information on our drawings.

Experimentally, for Playa Vista we added six new options to the three original "prototypical" ones which are depicted in the construction drawings. It was very easy to divide the slab, slope the garage floors, and add wall curbs where required.

The accurate depiction of each variation is important because of three stairs which may require adjustment, where headroom, landing depth and handrail extensions clearances are critical. Additionally, detailing around the base of the building must be coordinated where the lower slab offset may be as much as twenty-one inches below the main floor.