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How much sun does a house get — and how do I check before I buy?

The sun's position can be calculated precisely from address, date and time — what you have to measure yourself is the house's north direction, where the windows sit, and how high the neighbours and trees stand.

What can be calculated, and what has to be measured

Working out how much sun a house gets mixes together two entirely different questions. One is pure astronomy: where in the sky does the sun stand on 14 November at 2pm, at that exact address? That's a closed calculation — latitude, longitude, date and time go in, azimuth and solar elevation come out. There is no guesswork and no uncertainty in it.

The other question is local and can't be looked up anywhere: which way does the house face, where do the windows sit, and what's standing outside? Two identical houses on the same street can get wildly different light, because one is rotated 40 degrees, and because the neighbour's roof rises just south of the other one. That part you have to read for yourself, standing there.

So the useful division of labour is: let astronomy be astronomy, and spend your time on the north direction, the windows and the horizon. Those are what decide whether the living room gets afternoon sun in March, or sits in shade from eleven o'clock onwards.

The formula for the sun's height at midday

The sun's elevation above the horizon at its highest point of the day (solar noon, which isn't 12 o'clock on the clock) is found like this:

solar elevation = 90° − latitude + declination

Declination is the sun's tilt relative to the equator on a given date. It swings between +23.4° at the summer solstice (around 21 June), 0° at the equinoxes (around 20 March and 22 September), and −23.4° at the winter solstice (around 21 December).

An example with a house at 55.7° north latitude — roughly southern Scandinavia and northern Germany:

• Equinox: 90 − 55.7 + 0 = 34.3°
• Summer solstice: 90 − 55.7 + 23.4 = 57.7°
• Winter solstice: 90 − 55.7 − 23.4 = 10.9°

Move two degrees north and you subtract two degrees from all three figures; two degrees south and you add two. You can read your own latitude off any map — it's the only number you need to swap in to make the calculation match your address.

What matters here isn't the numbers themselves but the relationship between them: the winter sun stands at roughly a fifth of the height of the summer sun. A sun at barely 11 degrees comes in through the window almost horizontally, throws shadows metres long, and gets blocked by almost anything — a carport, a hedge, a neighbour's shed.

Where the sun rises and sets — not due east and west

A common misconception is that the sun rises in the east and sets in the west. That's only true two days a year, at the equinoxes. The rest of the time the rising and setting point shifts along the horizon, and the further you live from the equator, the more it shifts.

The direction at sunrise is found from cos(azimuth) = sin(declination) / cos(latitude), where azimuth is measured from north, clockwise.

For 55.7° north at the summer solstice: sin(23.4°) = 0.398, cos(55.7°) = 0.563, and 0.398 / 0.563 = 0.706. That gives an azimuth of roughly 45° — northeast. The sun sets correspondingly around 315°, in the northwest. At the winter solstice the sign flips, and the sun rises around 135° (southeast) and sets around 225° (southwest).

The consequence is tangible: a northeast-facing bedroom that's pitch dark from October to March gets direct morning sun in June. And a west-facing terrace that basks in evening sun in July gets nothing in December, because by then the sun sets far down in the southwest — behind the neighbour's house.

How to find the north direction of a house

North is the number the whole daylight study hinges on, and it's also the one people most often get wrong. Four ways to find it:

1. The north arrow on the floor plan. A property listing's floor plan usually has a north arrow. Note that the arrow rarely points straight up — if the drawing has been rotated to fit the page, it's the arrow, not the edge of the paper, that counts.
2. The map from above. Find the plot on an aerial photo or a map with north pointing up. Read off the angle between the house's long axis and the north-south line. That's the angle every sun calculation needs.
3. The compass in your phone. Stand with your back against an exterior wall and read which way the façade faces. Keep the phone away from car keys, radiators and reinforced concrete — magnetic things pull the needle off.
4. The sun itself. The sun sits due south at solar noon. If daylight saving is in effect, that's roughly 1pm on the clock, otherwise roughly noon — both shifted by how far east or west you sit within your time zone. See which way the shadows point at that time: they point north.

In HouseSense, the north direction is set by placing the house on the map, and the app then calculates the sun's path for that exact address and date. The sunlight in the 3D model is therefore genuinely calculated light, not a decorative lamp.

What casts shade: measure the angle, not the distance

The only thing you need to measure outside is an angle: how high above the horizon does whatever stands south of the house rise? If that angle is bigger than the sun's height on the date you care about, you're standing in shade.

The formula is shadow angle = arctan(height ÷ distance). An example: the neighbour's house is 8 metres to the ridge and stands 15 metres south of the living-room window. 8 ÷ 15 = 0.53, and arctan(0.53) = 28°.

A shadow angle of 28° has to be weighed against the sun's height on the dates that matter. At the equinox the sun stands 34° high at 55.7° north — above the neighbour's roof, so the living room gets midday sun. At the winter solstice the sun stands barely 11° high — well under the 28 degrees, so the living-room window sits in the neighbour's shadow all through December and January. Exactly that kind of thing can be settled with a calculator during the viewing.

Without a tape measure: a rule of thumb is that an object blocks the winter sun if it's more than roughly a fifth as tall as the distance out to it. An 8-metre height needs more than 40 metres of distance to fully clear an 11-degree sun.

The checklist for the viewing

• Read off the north direction before you go in. Write it down in degrees, not as "southwest-facing".
• Note which wall each window sits in — and how wide it is. Two small windows facing south give less light than one big one facing east.
• Look at what stands south, southeast and southwest of the house and guess the height. A neighbour's roof, a deciduous tree, a hedge, a garage.
• Remember the trees' season. A viewing in February shows just branches. The same branches are a dense screen in June.
• Take a photo facing south from every living room. It's the cheapest documentation of the horizon you can get.
• Look at roof overhangs and balconies above the windows. An overhang that blocks the high summer sun lets the low winter sun in — that's often an advantage, not a flaw.

Why "number of sun hours" is a poor figure to chase

It's tempting to ask for a single number: how many sun hours does the living room get? The problem is that the figure depends on the date, on where on the floor you measure, on how wide the window is, and on clouds — which nobody can predict. A single number hides everything that can actually be changed or chosen around.

The useful question instead is: on which dates and at which times does the sun hit through this specific window, and what's in the way? That can be answered precisely, and the answer points straight to an action — fell a tree, move the dining table, choose a different room for the home office, or don't buy.

With a measured floor plan and a correct north direction, you can answer that question for every room in the house at once. That's the exercise HouseSense's daylight study runs: the house's own geometry, the address's latitude and a date you choose yourself — 21 December at 1pm, say, when it hurts the most.