Explainer · what is a key plan
What a key plan is and why drawings use one
By Sumana Kumar · Published 20 Sept 2022 · Updated 16 Mar 2024
A key plan is the small, simplified diagram tucked in a corner of a drawing that shows where the detailed area you are looking at fits into the whole. When a building is too big to draw on one sheet at a useful scale, it gets split across several sheets — and a key plan on each sheet shades the portion that sheet covers, so the reader always knows which part of the building they are reading. It is a navigation aid, and on a large project it is what stops a reader getting lost.
This explainer covers what a key plan shows, why split drawings need one, how it differs from a location plan, how match lines tie the sheets together, and how to set up a clear key plan in CAD.
What a key plan shows
A key plan is a miniature, stripped-down outline of the whole building or floor, with the area covered by the current sheet highlighted — shaded, hatched or outlined. It carries just enough — the overall shape, maybe the grid, the highlighted zone — to orient the reader, and deliberately omits detail so it reads instantly at small size.
Its job is purely locational. It does not need dimensions or finishes; it needs to answer "where am I in the building?" at a glance. Placed consistently, usually near the title block, the key plan lets someone flicking through a thick drawing set immediately see which corner of which floor each sheet describes, without decoding sheet numbers.
Why split drawings need a key plan
A large building cannot be drawn at a readable scale on a single sheet — a big floor plate at 1:50 would need paper the size of a wall. So the floor is divided into zones, each drawn on its own sheet at a useful scale. The cost of that division is that any one sheet shows only a fragment, and the reader can lose track of how the fragments fit together.
The key plan solves this. By showing the whole and highlighting the part, it restores the context that splitting the drawing removed. Every sheet in the set carries the same key plan with a different zone highlighted, so the reader can mentally reassemble the building and understand how their sheet relates to its neighbours.
Key plan vs location plan
A key plan and a location plan sound similar but answer different questions. A key plan shows where a sheet's area sits within the building — it is about navigating the drawing set. A location plan, sometimes called a site location plan, shows where the building sits within its surroundings — the site, the streets, the neighbours — at a small scale, often for planning purposes.
So one zooms out to the wider world, the other zooms in to a part of the building. A drawing might carry both: a location plan to place the building in the world, and a key plan to place the sheet within the building. Keeping the two distinct avoids confusion about which scale of context is being shown.
Match lines and how sheets join
When a plan is split across sheets, the cut between zones is drawn as a match line — a clear line, labelled, that marks where this sheet's coverage ends and the adjacent sheet picks up. A note at the match line points to the sheet that continues the drawing, so the reader can follow the building from one sheet to the next without a gap.
The key plan and the match lines work as a pair. The match line says "continued on the next sheet" at the edge of the drawing; the key plan shows the same division as a whole, so the reader sees both the local join and the overall jigsaw. Together they make a split drawing navigable instead of a set of disconnected fragments.
Where the key plan sits and how big
By convention the key plan lives in a consistent corner of the sheet, usually adjacent to the title block, so a reader knows exactly where to look on every sheet. It is small — a thumbnail relative to the main drawing — because it only needs to convey position, not detail. The highlighted zone must stand out clearly against the rest of the outline.
Consistency is everything. The key plan should appear in the same place, at the same size, with the same outline on every sheet of the set, varying only in which zone is highlighted. That repetition is what lets the reader use it without thinking — the eye learns where the key plan is and reads the highlighted area automatically.
Setting up a key plan in CAD
In CAD the clean approach is to keep a single simplified outline of the whole building as a block, and place it in the layout near the title block on each sheet, with the relevant zone highlighted by hatch or a heavier outline. Because it is the same block everywhere, the outline stays identical across the set and only the highlight changes per sheet.
Draw it in paper space at a small fixed size so it does not compete with the main viewport, keep it on an annotation layer, and add the match-line references that tie it to the neighbouring sheets. Set up once as a reusable block, the key plan then drops onto every sheet quickly and consistently, giving the whole package a coherent navigation system.
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Questions
Frequently asked
What is a key plan on a drawing?+
It is a small, simplified diagram of the whole building or floor with the area covered by the current sheet highlighted. It orients the reader by answering 'where am I in the building?' at a glance, and it deliberately omits detail so it reads instantly at thumbnail size near the title block.
Why do drawings need a key plan?+
Because a large building can't be drawn at a readable scale on one sheet, so it is split across several. Each sheet then shows only a fragment, and a key plan restores the context by showing the whole and highlighting the part, so the reader can see how the fragments fit together.
What is the difference between a key plan and a location plan?+
A key plan shows where a sheet's area sits within the building, helping you navigate the drawing set. A location plan shows where the building sits within its surroundings — the site and streets — at a small scale. One zooms in to a part, the other zooms out to the wider context.
What is a match line?+
It is a labelled line marking where one sheet's coverage of a split plan ends and the adjacent sheet continues, with a note pointing to that next sheet. Match lines and the key plan work together: the match line shows the local join, the key plan shows the overall division.
How do I make a key plan in CAD?+
Keep a single simplified outline of the whole building as a block and place it on each sheet's layout near the title block, highlighting the relevant zone with hatch or a heavier outline. Using the same block everywhere keeps the outline identical across the set, with only the highlight changing per sheet.
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