Adding elevation data from contour lines to point layer using QGIS?

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I want to calculate the number of vacant lots within 300m from a given residential property, i.e. "There are 3 vacant lots within a 300m radius from property X." My source file containes both residential property and vacant lot data as point layers.

Here's a short description of my workflow. I've successfully completed steps 1 and 2. How do I complete step 3 ?

1. Create heatmap of vacant lots @300m radius.
2. Create a layer of contour lines based on the heatmap. The "elevation" of the contour lines ranges from 1 to 13.
3. Append "Elevation" attribute from contour line layer to residential properties layer.

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