Use the house setbacks, not the shed setbacks
Sheds and detached garages are accessory structures and often get a shorter side or rear setback. A room addition, a bump-out, or a second story over an existing wing is principal structure. It has to meet the setbacks for the dwelling.
Those distances are zoning, so they are not the same from town to town. A pattern you will see often in single-family districts is a deeper front and rear yard than the side yards, with a larger setback on a corner lot's street side. Write down the required front, interior side, street side, and rear yards from your district standards, plus the maximum lot coverage and height. Do not copy a neighboring city's table.
Some cities also require a licensed architect or engineer to seal the construction drawings for an addition, even when they will accept a clear plot plan from the owner. Ask before you assume the site plan is the whole submittal.
What reviewers look for on the plot plan
Residential addition checklists from building departments line up on this list:
| Element | Why it matters |
|---|---|
| Property lines, lot dimensions, and lot area | Coverage is a percentage of this number |
| Street name and adjacent right-of-way | Shows which yard is the front |
| Existing house, labeled, with distances to the lot lines | Context, and proof the house itself is legally placed |
| Proposed addition, labeled, with length, width, and height | The work being approved |
| Distances from the addition to each lot line and to other buildings | The setback check |
| Easements | An addition over a utility easement is a reject even inside the setback |
| What is being removed | A demolished porch or a wall that moves changes the footprint |
| Driveway and any new impervious area | Some codes cap impervious cover separately from building cover |
| North arrow, scale, address | So the sheet can be checked without a site visit |
If the addition is large, the same application may also want grading, erosion control, or a drainage note. That is a separate plan. The site plan still needs to show the footprint those sheets refer to.
Existing and proposed have to be obvious
The most common plotting mistake is one outline that mixes the old house and the new room. Reviewers want two graphics:
- Existing — the house, garage, and paving that stay. In ZoneMapper, mark these Existing so they draw with a dashed stroke.
- Proposed — only the addition. Leave it Proposed so it stays a solid stroke.
If you are removing a deck or a wall to build the addition, label that area “to be removed” instead of leaving it looking like new work. A second story over an existing footprint should say so. Otherwise the coverage math double-counts the ground floor.
Lot coverage
Add the existing roofed footprint and the new footprint, then divide by the lot area. Open decks, eaves, and detached sheds may or may not be in that total — the zoning definition decides. Impervious cover is often a second percentage and includes driveway, walks, and patios.
In ZoneMapper, shapes you draw start as impervious and count toward coverage. Set Surface in Layers when a patio or gravel area should be permeable, gravel, or not counted. Enter the district's maximum in the compliance fields so the pass/fail is on the sheet you export, not only in your notes.
How to draw it free in ZoneMapper (about an hour)
1. Open the addition template, a survey, or your address
Start from template and choose Addition for a sample house with a side addition, dimensions, and placeholder setbacks. Replace every sample number. A survey PDF is the better base when you have one. Address lookup loads aerial imagery and, where coverage exists, a GIS parcel outline you should still check against the survey.
2. Trace the lot
Use Lot boundary and Finish lot boundary if the outline is not already on the map. The lot area used for coverage comes from that boundary.
3. Draw the house, then the addition
Draw the existing house and mark it Existing. Draw the addition with Rectangle at its real width and depth and leave it Proposed. Label both.
4. Dimension the yards
Use Dimension from the addition to each lot line it approaches, and from the existing house where the checklist asks for those too. Enter the required setbacks in the Setbacks panel. If the red zone covers the addition, the plan fails the rule you typed in — move the addition or confirm you are in variance territory before you apply.
5. Export the plot plan
Fill in the title block and export the PDF. Submit it with the floor plan, elevations, and any sealed sheets your department requires.
Common reasons addition site plans are rejected
- The addition is drawn to accessory-structure setbacks
- Existing and proposed are one unlabeled blob
- A side-yard dimension is missing on the yard the addition actually enters
- Coverage math uses the lot area without the new footprint, or forgets an existing garage
- The addition crosses an easement that was on the survey and not on the drawing
FAQ
Is a site plan enough for a small bump-out?
Rarely for the building permit. Even a one-room addition usually needs a floor plan and a section so they can see headers, egress, and the foundation. The site plan is the zoning sheet in that set. Some places waive full design review for a minor addition that matches the house and stays inside the setbacks, but they still want the plot plan.
What if the existing house is already in the setback?
You generally cannot extend the nonconforming wall further into the setback. You can often build up or back within the legal envelope. That is a zoning question to confirm in writing, and it is a common reason to apply for a variance instead of a straight permit. Show both the existing encroachment and the proposed wall so the reviewer can see you did not make it worse.
Do second-story additions need a new site plan?
Yes. Even when the footprint does not grow, the plan should say the work is a second story, show the height, and confirm the footprint underneath. Height limits and sometimes floor-area ratios still apply.
More permit guides
ZoneMapper is a drafting aid. Setbacks, height, coverage, and whether construction drawings must be sealed vary by jurisdiction. Verify them with your building department before you submit or build.