1

Imagine a house/building, seen from outside, at sunrise. As the the sun moves in the horizon, a) the shadow of the house is displaced/deformed, and b) some faces of the house have more/less sunlight in the afternoon than in the morning.

I need to predict those changes along the day and along the year. Besides the external 3D shape of the house, this only depends on space & time. Space: the latitude, longitude and orientation of the house/building. Time: the hour, day, month of interest.

I have the exterior measurements of the house. At this point I treat the house as a solid object (ie I don't care about the light inside the house; I also don't care about cases where light which passes thorough the building eg two opposite windows).

I only need to predict two interrelated things (along the day/year) for planning and visualization purposes:

1) the shadow of the house

2) the faces of the house which will be receiving light

With (1) I can predict how the adjacent buildings/parks/spaces would be affected by sunlight decrease and with (2) I can plan to put more/less and/or smaller/larger windows in different places of each wall of the house.

I know that many years ago this was possible with a plugin for Google Sketchup, but for different reasons I would like not to use this option but a free application (if FOSS, even better). Given the recent surge of 3D modeling/design programs I thought maybe there are other programs which are capable of doing this.

3

I did a quick search using the terms "architecture modeling with blender sun shadows" considering that Blender (free software, very powerful, challenging to learn everything) allows for specific illumination placement and precise shadow adjustment as well.

The results were substantial, locating some architecturally focused web sites that will lead to other useful related pages. I've lost the specific link, but one post in comments on another site indicated that one can enter astronomical data into the lighting device position, suggesting that you can have precise placement for a specific time of year.

shadows v1

shadows v2

The above images are sourced from the previous link and show some of the variation permitted within Blender. The site itself may be of value for your purpose.

I've used Blender occasionally for 3D model creation and editing, but have not learned enough nor retained enough to create what you describe. The links from the above search indicate that it's certainly possible.

blender lighting and shadows with add-on

The above image came from a web site that appears to offer a non-free add-on to Blender to create more realistic shadows. You may have no objection to create results as shown in the 'before' image, which is why I've included it.

Allowing that you've used SketchUp to create similar models, I suspect the level of details shown in these images may be greater than required, but that only simplifies what you'd have to learn to make shadows perform as desired.

Blender has tremendous support on the internet, with many tutorial videos as well as a substantial user base and forums.

2

There is a open source software Blender to do this. It's mentioned by @fred-dot-u already, but my edit would have been too much, so this might look repetitive.

Features I consider relevant for you:

3D model import

You don't need to learn all parts, since it can import many 3D formats, so you can create e.g. the house in a totally different program. It seems to me that texturing is not needed for your task, so the import may result in a white house, but that's not relevant.

I can't exactly tell you which formats you can import. Here are the ones that are enabled by default on my machine. I see that the list of available formats in the options is even longer.

Import functionality

Rooms, Windows etc

There's a plugin that comes with Blender. It's called Archimesh. It needs to be activated.

See this Youtube video to see how it works. It's great if you want to change window sizes according to the sunlight.

Sizing

Blender supports metric and imperial scales, so you can resize your models to a consistent size:

Blender unit settings

Sun lamp

Blender supports a "sun" type of lamp:

Sun lamp in Blender

Sun simulation

There is a sun position plugin. You'll need to place that in the <Blender install dir>/scripts/addonsdirectory manually.

After a restart of Blender, activate it in the preferences:

Activating the sun position plugin

It works

I tried it in Blender 2.79 and it's quite simple to use. That said, I have attended 2 courses in Blender, I was able to build this in ~20 minutes (without Archimesh plugin):

Sun animation

Some final words

It will be very hard to make this photo-realistic.

It will be very hard to get the energy of the sun correct so that you can judge whether or not enough light comes through your windows.

I would have recomended the Architecture Academy by BlenderGuru. Unfortunately it has been retired for good reasons, though. Watch the trailer to see what's possible if you're a professional 3D designer.

The same applies for the Nature Academy, as well by BlenderGuru. It would have been a perect fit if you needed plants and trees in your simulation. Watch the trailer.

If you can afford buying some models, see

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