How Sightlines Affect Privacy

A view is a straight line

Light travels in straight lines, so what someone can see is determined by straight lines from their eyes. A barrier creates privacy only if it crosses every line between the viewer and what you want hidden. Everything else — height, width, placement — follows from that.

Similar triangles: the height at any point

Picture the line from an eye 5 m up to a point on the ground 10 m away. Halfway along, the line is halfway down: 2.50 m (8 ft 3 in). That is similar triangles, and it generalizes to any heights and distances:

height at distance d = eye height + (target height − eye height) × d ÷ total distance

Every calculation in the planner is built on this relation, applied in three dimensions: the “distance” is measured along each real sightline in plan, so windows offset to the side and rotated screens are handled correctly.

Which sightline matters: far edge or near edge

An area has many points. When the viewer is higher than what you hide, the sightline to the far edge is the flattest and crosses any barrier highest — it sets the height. When the viewer is lower, the near edge does. Because the crossing height changes steadily across the area, the extremes are always at its corners, which is why the planner checks corners rather than guessing.

The privacy shadow

Behind any barrier there is a zone that an elevated viewer cannot see, like the shadow a wall casts from a lamp. Its depth is:

shadow depth = viewer-to-barrier distance × (barrier height − hidden height) ÷ (eye height − barrier height)

If the eyes are below the barrier top, the shadow is unlimited; if what you hide is as tall as the barrier, there is none. A worked table is on the fence sightline calculator page. In the planner, the same idea gives the exact hidden and visible parts of your area. For example, a 2.2 m screen 8 m from a 4.6 m viewpoint hides 58% of a 4 m-deep patio starting just behind it.

View cones and width

Seen from above, sightlines from a viewer fan out to the corners of your area — a view cone. A screen must be wide enough to cross the whole cone at its position. Near the viewer the cone is narrow; near the area it is as wide as the area itself.

A window is not one point: anyone can look from either side of it, which widens the cone. In an example with the window 2 m to the side, treating it as a point needs a 3.78 m (12 ft 5 in) run; treating it as a 2 m-wide opening needs 4.18 m (13 ft 9 in). Enter window and balcony widths for this reason.

Elevation and slopes

Heights must be measured from a common reference. Raising the private area raises the line it must be hidden below, and lowering it does the opposite. With the viewer and screen unchanged:

• patio on level ground: 2.38 m (7 ft 10 in) needed;
• patio 60 cm higher (a raised deck): 2.77 m (9 ft 2 in);
• patio 60 cm lower (a sunken terrace): 1.99 m (6 ft 7 in).

Sunken seating is one of the few ways to gain privacy without building anything tall.

Several viewers: the limiting sightline

Each viewer has their own set of sightlines. A barrier must satisfy all of them, so the one that demands the most height at the barrier’s position is the limiting sightline. Moving the barrier can change which viewer is limiting — a key reason to plan with every viewpoint entered.

What geometry does not capture

• Partial opacity. Slats, lattice, foliage and glass block partly; the planner assumes a solid barrier.
• Movement. People lean over railings, stand on steps and move around windows.
• Change. Plants grow and lose leaves; neighbors build extensions.
• Rules. A geometrically required height is not a permitted height. Check local rules before building.

A safety margin and an on-site check with a pole cover most of this. The planner does the rest of the geometry for you.

Frequently asked questions

What is a sightline?

A sightline (line of sight) is the straight, unobstructed line between an observer’s eye and the thing they look at. If any solid object crosses it, the view is blocked.

Why does the far side of my patio need more screen height?

When someone looks down at your patio, the line to the far side is flatter than the line to the near side. At any point between them, the flatter line is higher, so it is the one a screen must clear. From below, the near side is the demanding one.

Does distance make people harder to see?

Distance makes details harder to make out, but it does not block a view. The planner treats any unobstructed sightline as a view, regardless of distance, so results do not depend on eyesight, lighting or attention.