Sun in the new storey — how bright Vienna’s added storeys would be
On 12,319 houses in Vienna with room for another storey, the new storey would get 6.5 hours of direct sun on 21 December on average — the first floor below it 5.4. Calculated for every house from the official eaves heights of its neighbours.
6.5 h
direct sun on 21 December in the new storey, on average — 5.4 h on the first floor below
“What Vienna still allows” counts where the building class would still permit a full storey. This page asks the next question: how bright would it be? For each of those houses the calculation builds the horizon from the measured eaves heights of all neighbours within 150 m — once seen from the window height of the new storey (1.5 m above today’s eaves), once from 4.5 m, roughly the first floor. Then the sun runs across it on three reference days. It is the same calculation Azima runs for a single address, here for 12,319 houses: 78 % of all houses with headroom — for the other 3,390 the roof model does not know the neighbourhood completely, and half a neighbourhood would invent sun.
Three reference days
Mean direct sun in the new storey; the first floor (4.5 m above ground) in the small print of each row.
What the new storey gains in December
586 of the calculated houses have a first floor that gets no direct sun at all on 21 December — in 292 of them the new storey would get at least two hours. That is the difference a storey makes in a Viennese street: not more floor space alone, but light where there was none.
Where the gain is largest
Among the districts with at least 100 calculated houses, the new storey gains the most December sun over the first floor in Margareten, Brigittenau and Leopoldstadt (3.4 h, 3.0 h, 2.8 h): dense streets, where the first floor sits in the shadow of the house opposite and the new storey looks over it. In Donaustadt, Liesing and Floridsdorf the difference is smallest (0.3 h, 0.4 h, 0.5 h) — low-rise, open, the first floor already gets its sun.
All 23 districts
Mean gain in direct sun on 21 December, new storey against first floor; houses calculated of all houses with headroom. Districts under 30 calculated houses carry no figure.
What this figure is not
- The sun at one point above the house, seen all round — not the sun at one window. A storey has several sides; the orientation of each window decides.
- The window height of the new storey is an assumption: 1.5 m above today’s eaves. A storey that sits lower or higher sees a different horizon.
- Only buildings shade: the horizon is built from eaves heights. Trees, terrain and the roof shapes of the neighbours are not in it — on a hillside or under old trees the real light differs.
- Only where the roof model knows the whole neighbourhood: 3,390 houses with headroom stay without a figure rather than getting a guessed one.
- Whether the storey may be built is decided by the development plan, the protection zone, the structure and the owners — this figure knows none of them.
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Frequently asked
How much more sun does a new storey get than the first floor?
Across 12,319 houses, on average 1.1 hours more direct sun on 21 December (6.5 against 5.4 hours); the median is 0.3 hours. In June it is 14.4 against 13.4 hours — the high summer sun clears most neighbours anyway.
Why December?
On 21 December the sun stays lowest — in Vienna about 18° at noon. Every neighbouring eaves counts then. A storey that gets sun in December gets it all year; a first floor that stays dark in December is dark for months.
Does this mean my attic flat would be bright?
No. The figure is the sun at one point above the house, seen all round. A storey has windows to several sides; how much a single window gets depends on its orientation. Azima calculates that for one address and, from a floor plan, for each room.
How was this calculated?
For each house with room for at least one full storey (as in “What Vienna still allows”): the horizon from the official eaves heights of all neighbours within 150 m (roof model LOD2.1), seen from 1.5 m above today’s eaves and from 4.5 m; direct sun on 21 December, 21 March and 21 June in 5-minute steps. Only where the roof model knows the neighbourhood completely and at least 8 neighbours form the horizon.
Sources: Stadt Wien — Generalisiertes Dachmodell LOD2.1 (CityGML), data.wien.gv.at, CC BY 4.0 · Stadt Wien — Generalisierte Flächenwidmung und Zonen (OGD-WFS), Abruf 2026-09-23, data.wien.gv.at, CC BY 4.0 · Stadt Wien — Bezirksgrenzen, data.wien.gv.at, CC BY 4.0. Edited: per-house calculation by Vienna Living Map (scripts/generate-sonne-wien.mjs), status 2026-10-09. Not legal advice — the City of Vienna's plan document is authoritative.
What Vienna still allows · Light and sun · Green roofs · All districts