Changing to alternate label if first label does not fit in QGIS?

Here's a somewhat approximate (but hopefully effective) way to do it.

First some math. We need to figure out how many characters at a certain font-size a feature can contain. Here things / assumptions to know:

• assuming metric units, (added slight change below which may make this work for us foot crs.)
• font size is a measure of font height. Most font's are about half as wide as they are high.
• using pt for font height, 1pt is roughtly 0.035CM
• assuming your parcels are roughly rectangular / regular

To find how long an item is on screen / print we can use it's longest dimension (length or hight, or just length if your labels are horizontal only) / scale * 100 (to go from M to CM)

Then we can see if our label would fit.

You could do it all in the expression engine, but a custom python expression would be more efficient. Here is a tutorial on how to create one.

My custom expression:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def labelFits(labelStr, fontPt, scaleM, feature, parent):
    # returns true if a label will fit in the feature at a given
    # font size and scale

    bbox = feature.geometry().boundingBox()

    # for CRS in us foot
    # fontWidth = fontPt * 0.0875

    # for CRS in metric
    # 1pt = 0.035CM
    fontWidth = fontPt * 0.035 * 0.5

    # length of maximum dimension on-screen/print in CM
    labelRoom =  max(bbox.width(),bbox.height()) / scaleM * 100

    # approx length of our label in CM based on font height * 0.5
    labelLen = len(labelStr) * fontWidth

    if labelRoom > labelLen:
        return True
    else:
        return False

Then just use that expression in the label:

if(labelFits( "yourAttribute" , 8 , @map_scale ), "yourAttribute",$id)

You'll need to plug in whatever attribute you're actually using of course, and change the font size from 8 to whatever you're using. Also, since fonts vary quite a bit you may need to tweak it a little bit till it looks right.

You could also use that expression to do things like change the color based on if the label fits or not.

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To use this expression outside the map canvas (i.e. in a composer label or attribute table) we need a way to programmatically get the scale of a given map item. I've posted code to accomplish that in another answer here The method there gives us an expression getScale('composername','mapname') so to incorporate it into our expression:

if(
    labelFits( "yourAttribute" , 8 , getScale('composername','mapname')),
    "yourAttribute",$id)

Duplicate the layer, one with no symbology, and set different zoom levels/obstacle settings for the labels for each.