Architecture and construction
How to simulate shadows from a planned building
A planned extension, garage or new block can shade gardens and windows well beyond its footprint. Construction Mode in SunCast puts a simple mass on the 3D map so you can watch its shadow through the day and the year.
By SunCast team Last updated: October 14, 2026
Short answer
Open the site on SunCast’s 3D map, switch to Construction Mode and place a block where the structure will stand. Set its width, depth, height and rotation, then step through the day on 21 December, the March equinox and 21 June. Note which windows, gardens or roofs the shadow reaches and at what times.
How far a two-storey shadow reaches
Height and season set the reach. A 6 m block in Leeds, about the height of a two-storey extension, casts a noon shadow of 26.5 m on 21 December, when the sun is 12.8° high. At the March equinox (36.0°) the noon shadow is 8.3 m, and on 21 June (59.6°) it is 3.5 m. Noon gives the shortest shadow of the day: at 10:00 on 21 December the same block’s shadow is about 41 m long (calculated with SunCast’s sun calculator).
Direction changes as much as length. On 21 June the block’s shadow points west (278°) at 09:00 BST and east-north-east (64°) at 16:00. A neighbour’s garden that is clear at noon can sit in shade all afternoon, and a drawing for a single time of day will not show it.
Use the preview to spot obvious problems and compare options, such as a lower roofline or a different position, before paying for drawings. Then take what you found to the architect or planning officer.
Running a shadow check in Construction Mode
1. Find the site on the 3D map
Search the address and zoom in until the plot, the neighbouring houses and their gardens are easy to see.
2. Place a block
In Construction Mode, drop a virtual building where the structure will stand. A plain box is enough for a first check.
3. Match the planned size
Set width, depth and height from the plans or your best estimate, and rotate the block to line up with the boundary. Using the ridge height of a pitched roof gives you the worst case.
4. Step through one day
Move the time from morning to evening and note the hours when the shadow crosses a neighbour’s window, patio or garden bed.
5. Repeat on three dates
Check 21 December, the March equinox and 21 June. December gives the longest shadows; the equinox is a fair middle case for spring and autumn use of a garden.
6. List what needs a professional answer
Write down the affected windows, beds or solar panels with the dates and hours involved. An architect or planning officer can do more with that list than with a screenshot.
What SunCast adds
You test the planned mass on the real site instead of imagining its shadow from a flat drawing.
| SunCast feature | How it helps |
|---|---|
| Construction Mode | Place a building block on the map and set its width, depth, height and rotation. |
| Date and time | Move through hours and seasons to see when the new shadow matters most. |
| 3D map context | The planned block shown against mapped buildings and terrain around the site. |
| Sun Times & Position Data | Azimuth, elevation and shadow ratio for the moment you are viewing. Multiply the block’s height by the shadow ratio to get its shadow length. |
Mistakes to avoid
Testing one date
Check the solstices and the months when people use the affected space, not one sunny day.
Forgetting height
When the sun is low, an extra metre of height can add several metres of shadow.
Treating a preview as approval evidence
Formal approvals need the studies and drawings your local process asks for.
Limits
Construction Mode models a simple block, and the surrounding shadows depend on mapped building data, so trees, fences and recent work may be missing. Use it for preliminary review. It is not a certified planning, surveying, engineering or permitting tool.
Check it in SunCast
Planning an extension or a new build? Place it in SunCast’s Construction Mode and check the December shadow before the formal design work starts.
Frequently asked questions
Can SunCast simulate a future building?
Yes. Construction Mode lets you place a virtual building block, set its size and rotation and preview its shadows for any date and time.
What dates should I check for building shadows?
Check 21 December, an equinox, 21 June and the months when the affected space matters most. Winter usually gives the longest shadows.
Can I use this for a planning application?
Use SunCast for early review and for explaining a concern. A planning application needs the professional studies, drawings and local process it requires.
