# Will an Extension Block My Sunlight? A Practical Shadow Check

Check how a proposed extension could affect sunlight with its height, direction and season. Use a simple shadow calculator before commissioning formal advice.

Canonical HTML: https://suncastapp.com/blog/will-an-extension-block-my-sunlight
Last updated: 2026-09-22

Author: **SunCast team**

An extension can block sunlight when its shadow reaches a particular window, patio or garden area at a time you care about. The useful first check is not whether the extension casts *a* shadow—every opaque structure does—but whether that shadow reaches the affected place on the relevant dates and hours.

Start with the proposed height, its position relative to the space, and the local sun direction. The [SunCast Shadow Length Calculator](/tools/shadow-length) turns the height and solar elevation into an illustrative ground-shadow length. It is a screening tool for a simple vertical object on level ground, not a planning assessment or a model of the proposed building.

> **Quick answer**
>
> A proposed extension is most likely to affect sunlight when it sits between the sun and the window, patio or garden you are checking. Compare its height, the sun’s direction and its shadow length across the dates and hours that matter. Check more than one season before deciding that an effect is minor.

## Try a first extension-shadow check

Search for the property, set the proposed height and move through the relevant time window. The result shows the ground-shadow direction and length for a simple block. Measure the distance from the extension’s relevant edge to the point you care about, then compare it with the result only when the shadow points towards that point.

[Explore a proposed extension’s shadow](https://suncastapp.com/tools/shadow-length)

SunCast is useful at this early stage because it makes a geometric question visible: where could a proposed mass cast its shadow as the sun moves? For a fuller exploratory view, SunCast Construction Mode can place and rotate a simple 3D building volume on a map. Neither view substitutes for the drawings, methodology or professional analysis required by a planning authority or a contractual decision.

## Start with the exact question

“Will it block my sunlight?” is too broad to test. Define the receiving point and the time that matters:

| Question | Better test |
| --- | --- |
| Will the kitchen lose morning light? | Check the window area during the breakfast hours you use it. |
| Will the patio be shaded at dinner? | Check the seating area over the intended evening window. |
| Will vegetables lose growing light? | Compare the bed’s direct-sun windows during the growing season. |
| Will a neighbour’s room be affected? | Identify the relevant façade, window and local process before relying on a preliminary result. |

An extension may not affect a whole garden or room in the same way. A corner of a patio can enter shade while the table remains sunny; a low sun can be blocked at one window while another floor is unaffected. Mark a point—or several points—on a simple plan before using any calculation.

## The three things that determine the shadow

### 1. The extension’s position and height

For an early check, use the height of the edge most likely to create the shadow. A flat roof, ridge, parapet and eaves can produce different edges. A taller extension usually produces a longer shadow at the same solar elevation, but height alone does not tell you where the shadow goes.

### 2. The sun’s direction

Solar **azimuth** is the compass direction of the sun, measured clockwise from north. The shadow runs in the opposite direction. Use the [Sun Calculator](/tools/sun-calculator) to inspect that direction for a chosen date and local time; an extension must actually lie between the sun and the affected place to block direct sunlight.

### 3. The sun’s height above the horizon

Solar **elevation** is the sun’s angle above the horizon. For a vertical object on level ground, the simple shadow-length relationship is:

**shadow length = object height ÷ tan(solar elevation)**

Low sun makes long shadows. This is why a design that seems harmless at midday can matter in the morning, late afternoon or winter. NOAA’s [solar glossary](https://gml.noaa.gov/grad/solcalc/glossary.html) defines azimuth and elevation; its [calculation notes](https://gml.noaa.gov/grad/solcalc/calcdetails.html) also explain why atmospheric conditions and horizon assumptions limit apparent precision near sunrise and sunset.

## A four-step first review

1. **Sketch the proposal and receiving area.** Mark north, the existing building, the proposed footprint, relevant windows and the patio or garden point. Add the proposed height and any roof edge that changes the silhouette.
2. **Choose realistic dates and hours.** Test the actual period the space is used. Add a lower-sun and higher-sun season; Earth’s tilt changes the path and height of the sun through the year, as explained by [NASA’s seasons overview](https://science.nasa.gov/helio-and-you-seasons-on-earth-mars-and-beyond/).
3. **Check alignment before distance.** First establish that the shadow points toward the receiving point. Then compare the calculated length with your measured ground distance. A long shadow in the wrong direction has no effect there.
4. **Record uncertainty and escalate where needed.** Note unknown roof dimensions, slopes, trees, changed ground levels and map gaps. Those are reasons to seek the correct professional assessment, not reasons to round an estimate into certainty.

## A worked, illustrative example

Suppose the top edge of a proposed extension is 3 metres above a level patio and the patio point is 5 metres away in the shadow direction. The table only tests length; it assumes the extension is wide enough and correctly aligned to shade that point.

| Solar elevation | Ground-shadow length from a 3 m edge | Reaches 5 m? |
| --- | ---: | --- |
| 45° | 3.00 m | No |
| 35° | 4.28 m | No |
| 31° approximately | 4.99 m approximately | At the boundary |
| 20° | 8.24 m | Yes, if aligned |

The threshold is approximately **arctan(3 ÷ 5) = 31°**. That does not establish when the patio will be shaded at a real address: it says only that a 3-metre vertical edge can reach five metres on level ground when the sun is low enough and the shadow is aligned. A sloping garden, a stepped terrace, a pitched roof, the width of the extension and sunlight passing around an edge all change the practical result.

## What this first check cannot answer

The embedded calculator deliberately leaves out several features of a real site:

- It does not load a proposed footprint, roof shape or neighbour’s window.
- It assumes level ground and a vertical object.
- It does not account for trees, clouds, existing obstructions or sunlight reflecting indoors.
- It does not apply a local planning policy or any daylight-and-sunlight assessment standard.

Use the result to ask better questions. If it suggests a possible effect, collect dimensions, dates, drawings and photographs before speaking with an architect, planner or other appropriate professional. If it does not suggest a simple alignment, still check the unusual features the model cannot see.

For a related home workflow, read [when a patio will be in shade](/blog/when-will-my-patio-be-in-shade). For the broader seasonal context, see [how shadows change from winter to summer](/blog/shadows-change-winter-summer).

The sensible next step is to run the simple check for the real address and the times the space is used. Treat the output as a transparent first estimate, then use the right professional process for any decision that depends on it.

## FAQ

### Can a shadow calculator tell me whether an extension will block my sunlight?

It can provide a first estimate for a vertical object on level ground: whether its shadow is long enough and points towards the area you are checking. It cannot replace a surveyed model of a particular extension, garden, window or neighbouring property.

### Which dates should I check for an extension shadow?

Check the dates when the affected space matters most, plus contrasting low- and high-sun seasons. In the Northern Hemisphere, a winter check often reveals longer shadows; the equivalent seasonal pattern is reversed in the Southern Hemisphere.

### Do I need a professional daylight or sunlight assessment?

Use a simple check to identify questions early. Follow the local planning process and obtain the appropriate professional advice where an assessment, application, neighbour agreement or certified result is required.