How to read a site plan
The drawing is what you are buying, what you are consented for, and what the contractor prices. Here is what all of it means.
Most self-builders learn to read elevations quickly — they look like the house. Site plans take longer, because they are a coded document rather than a picture, and a lot of what matters is in the annotation rather than the shapes.
It is worth learning, because the site plan is where the expensive commitments live. Here is a walk through the things that carry meaning.
The red line and the blue line
On a planning application, the red line marks the application site — the land the proposal relates to. It must include everything needed to make the development work, notably the access. The blue line, when present, marks other land in the same ownership nearby.
Check the red line against your title plan. They should agree. Where they do not, either the drawing is wrong or you do not own something the scheme depends on, and both are worth finding out about before submission rather than after.
Scale, north, and whether they agree
A site plan should carry a stated scale, a scale bar, and a north arrow. Do not take the stated scale on trust — measure a known dimension against the scale bar and see whether they agree. Drawings get rescaled when they are exported, printed to a different paper size, or dropped into a report, and a scale bar that no longer matches the dimensions is more common than it should be.
North matters for more than orientation. It determines overshadowing, solar gain, which rooms get morning light, and whether your neighbour's objection about loss of light has any substance.
Levels: AOD, EGL and FFL
Spot levels are usually shown as a small cross with a number, given in metres above Ordnance Datum. EGL is existing ground level; FFL is finished floor level. The relationship between them is one of the most consequential numbers on the drawing.
If the finished floor sits 450mm above existing ground, that difference has to be made up in fill, in foundation depth, and in the approach to the front door — which then has to be accessible, which means a ramp or regrading, which takes space you may not have. Small numbers here become real construction.
The access
The flared shape where a drive meets the road is a bell mouth, and the curves are kerb radii, usually annotated with an R and a dimension. The dashed triangle running along the road is the visibility splay, given as two numbers: the X distance back from the carriageway edge, typically 2.4m, and the Y distance along it, set by the speed of the road.
The Y distance is often longer than the drawing, which is why it frequently runs off the edge of the sheet with a note. At 30mph it is commonly 43m in each direction. Check it is achievable over land you control — a splay drawn across a neighbour's hedge is a negotiation, not a design.
Trees
Trees are drawn as a crown outline with a cross at the centre, sometimes with a dashed circle around them. The dashed circle is the root protection area: the zone within which excavation, compaction and level changes are restricted. Trees marked for removal are usually dashed or crossed.
A tree with an RPA overlapping your proposed footprint is a design problem, not a landscaping one. And a tree with a preservation order on it cannot be removed to solve that problem.
The title block
Bottom corner, usually. It carries the drawing number, the revision letter, the scale and the paper size. The revision letter is the part people ignore and shouldn't — if your contractor is pricing Rev A and your planning consent was granted on Rev C, you have a problem that will surface at the worst possible moment. Every drawing on site should be the current revision, and somebody should be responsible for making sure of it.
What is not on the drawing
A site plan shows the proposal. It does not show ground conditions, service capacity, covenants, or what the neighbours think. It shows where the drainage runs, not whether the ground will accept it. Read it for what it commits you to, and keep a separate list of the things it is silent about.