Engineer Site Plan - Lesson 1

When you discuss a project with someone, it will usually center around viewing the site plan. It's the skeletal structure of your project. It defines where everything is located. A proper Engineer Site Plan will comply with many regulations and guidelines set forth by the local jurisdiction and the Fire Department, as well as national requirements such as the Americans with Disabilities Act.

Designing and drafting the site plan accurately from the beginning will eliminate many problems encountered later in the project cycle as well as during construction. For the purposes of this discussion, the site plan will contain all of the proposed features on the surface with exception to any utilities (those will come later).

Many site plans are first designed by the project Architect. They create the vision of the project in conjunction with the client. Often it will go through several iterations before the Civil even gets involved. If, instead, you are tasked with this, you may get some useful points from reading our posts on this topic, but designing a site plan from scratch requires a bit of an artistic touch.

You will probably start with a preliminary site plan that the client already likes but was designed by an Architect. No offense intended to Architects, but they often do not take the time to draft the site plan with all the proper setbacks and dimensions. Your job is to make sure all the features are spaced appropriately (setbacks, minimum building separations, etc.) as well as drawn parallel and perpendicular where appropriate. Let's assume we are laying out a site plan for an apartment complex with a couple internal streets, parking bays, and a couple of large buildings.

The first step is to ensure you have an accurately drawn project boundary. Boundary research is outside of the scope of this blog, so please check with a licensed surveyor either within your company or already selected for your project. Not starting with an accurate site boundary means you will need to draw all or a portion of the site again, so this is a critical step.

Before we begin to draw our site plan, we must also determine some regulatory requirements:

• Minimum street widths
• Minimum inside curve radius, minimum curb return radius
• Minimum sidewalk widths
• Front, rear, and side building setbacks
• Minimum parking space widths and depths (covered in following post)

For the second step, you should find what the minimum street widths are as required by both your local jurisdiction as well as the Fire Department. You must use the wider of the two minimum widths. The Fire Department will also have information about the smallest radius allowable for curb returns on your project. These curves are based on their fire truck capabilities (typical radius is 20-ft on the inside). While you are researching this, also see if you can find the required sidewalk widths. Typical is 4-ft wide, but some Fire Departments and/or Cities want 5-ft or even 6-ft wide sidewalks.

The third step is to research the building setbacks according to your local jurisdiction. This information can be found in your City's municipal code, general plan, specific plan, or other government resources. Usually there are different setbacks for "front," "side," and "rear" though you may find many more complicated setbacks for different types of structures and also regarding what is adjacent to your property. Choose a side and offset the property line the correct setback amount. No building can be closer to this property line than this distance. Do this for all the property lines including any internal lot lines.

The fourth step is to choose one side of the project to align most of the buildings, curb, sidewalk etc. Usually, you will choose the longest side adjacent to the public right-of-way. The main reason for this is the site boundaries are usually not parallel on opposite sides, and perpendicular along the other two. If your project is perfectly squared, then your job is a little easier.

Next, begin offsetting lines in one direction (let's call it East-West), demarcating the sidewalk, curb, parking spaces, and centerlines of your internal streets. For the North-South direction, we don't necessarily want to be parallel to the property line. In most cases, it's easier to make your lines perpendicular internally and allow a little "slop" along your perimeter to account for the fact that the property lines are not square.

Offset the N-S property lines the appropriate building setback, but then draw a line that is perpendicular to your E-W lines but no closer to the property line than the setback line. Continue offsetting lines in the N-S direction as well. Use the Architectural Site Plan as a guide, but ensure you are using required minimum distances. Usually these distances will be whole numbers, not fractional numbers. The exception are landscape areas, which may be an odd distance.

When two streets intersect, the curb at each corner is rounded (called a Curb Return). Connect the intersecting streets with your curb return radius. Usually the outside radius (if any) is found by adding the minimum curb return radius plus the minimum street width. In our case we have parking spaces lining our streets, so we will ignore the outside radius.

At this point you can place your buildings within the building setback and sidewalk lines on your site plan. Now is a good time to double check that your linework is uniformly parallel and perpendicular.

Check back for the next post on this topic when we'll help you add the finer detail to the site plan including parking stalls, sidewalks, driveways, and curb ramps.

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