Winter Sunlight and the Distance to the House Next Door: Calculating How Far Away a Building Must Be for Sun to Reach Your Window

Check sunlight at the winter solstice

The sun is at its lowest of the year at the winter solstice (around 22 December). In Tokyo, the noon solar altitude at the winter solstice is about 30.9°, less than half the roughly 77.8° at the summer solstice. The lower the sun, the longer the shadow of the building in front, so if sun reaches a spot at midday on the winter solstice, you can expect it to reach it at midday on every other day of the year too.

Winter is also when you want sunlight most. Winter sun warms rooms, dries laundry and reduces condensation and mould. A building in front that didn't bother you at a summer viewing often becomes a problem only in winter, because shadows are about 7 times longer in winter than in summer.

The formula for the distance at which sun reaches a window

When there's a building in front of (south of) a south-facing window, whether sun reaches the window at midday on the winter solstice depends on whether the angle of the line from the window to the edge of the building's roof is lower than the winter solstice noon altitude. Converted to a distance, this gives the following formula.

The key is to think in terms of the "difference" from the window's height, not the height of the building in front itself. With the same two-storey house (7 m) in front, the difference is 5.5 m for a ground-floor window (1.5 m above ground), 2.5 m for a first-floor window (4.5 m) and 7 m for the garden ground (0 m).

This distance is the threshold for "whether sun reaches the window around noon on the winter solstice". At exactly the threshold distance, sun reaches the window for only a very short time around solar noon. If you also want sun in the morning and afternoon, you need more distance.

Hours of sun on the window by distance (Tokyo, winter solstice)

Using the Sun Position & Shadow Length Calculator, we worked out the hours of sun on a window in 5-minute steps under these conditions: Tokyo (Shinjuku), 22 December 2026 (winter solstice), a south-facing window, and the building in front running parallel to the window and long from side to side. With nothing in front, a south-facing window gets sun for 9 hours 40 minutes on the winter solstice, from 6:50 to 16:30.

With a two-storey house in front and a ground-floor window, sun finally reaches the window at 10 m, just past the required 9.2 m or so, and only for 3 hours 20 minutes around midday. At 12 m it exceeds 5 hours, and at 20 m it's about 80% (7 hours 40 minutes) of what you'd get with no building in front. A first-floor window, on the other hand, gets more than 6 hours even only 6 m away. In the same house, winter sunlight on the ground floor and the first floor can be completely different.

• If a ground-floor window faces a two-storey house to the south within 8 m, no direct sunlight gets in all day around the winter solstice
• With a three-storey building (10 m) in front, a ground-floor window needs 15 m or more of distance to get winter sun
• Garden ground is lower than the window, so it's even harder: with a two-storey house in front, 1 hour 50 minutes at 12 m and 5 hours 20 minutes at 15 m
• Even a block wall (1.8 m) shades the ground just north of it: 5 hours 30 minutes at 4 m away, 7 hours 30 minutes at 6 m

How window direction changes sunlight

You can also calculate for windows that don't face due south. Again for Tokyo on the winter solstice, with the building directly in front of and parallel to the window, we compared a 7 m building, 10 m away, with a ground-floor window (1.5 m).

With nothing in front, south-facing gets sun the longest, but with a building directly in front, southeast and southwest get more. That's because when the sun is off to one side of the window, its angle in the vertical plane of the window's direction (the profile angle) is higher than its actual altitude, so it clears the building in front more easily. A southeast window gets angled light in the morning, and a southwest window in the afternoon.

The distance needed varies by region

The winter solstice noon altitude gets lower the further north you go, so the same building requires more distance in northern regions. Here are the distances at which sun reaches the spot at midday on the winter solstice with a two-storey house (7 m) in front, by city.

For a south-facing ground-floor window 10 m from a two-storey house, sun on the winter solstice lasts 3 hours 20 minutes in Tokyo, 4 hours in Osaka, 4 hours 35 minutes in Fukuoka and 6 hours 55 minutes in Naha. In Sapporo the distance falls short of the roughly 12.6 m needed, so no sun reaches the window around the winter solstice. The further north you are, the more the distance between buildings matters for winter sunlight.

A checklist for viewings and land hunting

1. 1

   Check the window's direction

   Use your phone's compass to measure which way the window faces. A compass points to magnetic north, which in Japan is 5 to 10° west of true north. For how to correct it, see True north vs magnetic north.

2. 2

   Estimate the height of the building in front

   One storey is about 3 m. For a two-storey house, take about 7 m at the highest point of the roof; for three storeys, about 10 m; for an apartment block, the number of floors × 3 m.

3. 3

   Measure the distance to the building in front

   Use the distance tool in a map app to measure the horizontal distance from the window to the wall of the building in front. If pacing it out, checking your own stride length once (about height × 0.45) reduces the error.

4. 4

   Set the window height

   As a guide, a ground-floor window is about 1.5 m (floor height plus the middle of the window) and a first-floor window about 4.5 m. To find out about sunlight on a balcony floor, enter the balcony floor height.

5. 5

   Enter it in the Sun Position & Shadow Length Calculator

   Choose the location and enter the height of the building in front, the distance, the window height and the window direction, and the hours of sun on your chosen date and on the winter solstice are shown side by side as timelines. Share the result with family using "Copy URL with these settings".

If a house is going to be built on the empty lot next door, the building plan notice board on the site may show the building's height and number of storeys. Enter that height and the approximate distance from the boundary to the building to estimate winter sunlight after it's built.

Limits of the calculation, and how it differs from shadow regulations

• The building in front is assumed to be long enough to the left and right: in reality, sun comes round the sides, so narrow buildings or gaps between buildings give more sun than calculated
• Reflected light and skylight aren't included: even without direct sunlight, light reflected off white walls and from the sky brightens a room
• Trees, utility poles, balcony railings and eaves aren't included: the eaves or balcony of your own house may shade the lower part of the window
• Ground level differences: if your plot is higher or lower than the building in front, add or subtract the difference from the window height

To see how far shadows reach at each time of day, see How to calculate shadow length; for how to work out the winter solstice noon altitude itself, see How to find the noon solar altitude. How to draw direction lines on a floor plan is useful for marking directions on a floor plan.

FAQ

How far from the house next door do I need to be to get winter sun?

As a guide, distance needed = (height of the building in front − height of the window) ÷ tan(winter solstice noon altitude). In Tokyo, with a two-storey house (7 m) in front, that's about 9.2 m for a ground-floor window (1.5 m) and about 4.2 m for a first-floor window (4.5 m). This is the threshold for sun at midday, so if you want sun in the morning and afternoon too, you need to be further away.

Can a south-facing ground-floor room get no sun in winter?

Yes. In Tokyo, if a two-storey house to the south is within 8 m, almost no direct sunlight reaches a ground-floor window around the winter solstice. By calculation, you get about 3 hours around midday at 10 m and about 5 hours at 12 m.

Which month should I check sunlight in?

The basic approach is to check under winter solstice conditions (around 22 December), when the sun is lowest. The noon altitude barely changes for about two months around the solstice, so if there's no sun at the winter solstice, it will be similarly hard to get from late November to late January. A summer viewing alone won't tell you about winter sunlight.

Which gets more winter sun, southeast-facing or south-facing?

With nothing in front, south-facing gets sun for longer (9 hours 40 minutes at the winter solstice in Tokyo, against 7 hours 55 minutes for southeast). But with a building directly in front, southeast and southwest windows, where the sun comes in at an angle, clear the building more easily: with a 7 m building 10 m away, south-facing got 3 hours 20 minutes against 4 hours 35 minutes for southeast.

Can I use the results to check shadow regulations?

No. Shadow regulations set the areas covered, times and measurement height by local ordinance, and compliance is judged by drawing shadow diagrams based on the building's shape. This tool gives a guide to how much sun reaches a window or balcony.