Dissolving features based on another layer in QGIS

Question

I have two layers, see image below:

• Yellow: the ground where people can build;
• Red: existing buildings.

Create a shapefile of all grounds where >50% of the grounds are free.

In order to do this, I am wondering whether or not it is possible to dissolve the red features based on the yellow layer. In other words: I'd like to join/dissolve the red features within a yellow feature. If that were possible, a simple area (red) / area (yellow) * 100 would do the trick to calculate the area in use.

Calculating area (red) / area (yellow) * 100 without joining/dissolving red leads to an outcome that is not easily understandable.

Any ideas?

Answer 1

You can use the tool Overlap analysis:
Processing toolbox > Vector Analysis > Overlap analysis

This will give you the total area and percentage of each parcel covered by buildings.