"Mother, any distance" uses alliteration sparingly, meaning it feels all the more powerful when it does appear. The first example is in lines 1 and 2:
... distance greater than a single span
requires a second pair of hands.
This /s/ sound works subtly here, but gives the lines a sort of stretchy feeling; the /s/ sound "spans" across both lines (an effect bolstered by the consonance of "distance"), anticipating the poem's central image of the tape measure.
Another important moment of alliteration appears in line 6, when the speaker relates how the mother is noting down the measurements as the speaker unfurls the "spool of tape." The speaker characterizes this as "reporting metres, centimetres back to base." The precision of the two /b/ sounds gently evokes the process that the mother and speaker are undertaking—they're trying to take precise measurements of the house. The alliteration here also focuses readers' attention on this phrase, which positions the mother as a steady, anchoring home "base" for the speaker.
Lines 9 to 10 use alliteration too with the phrase "climb / the ladder to the loft." The three /l/ sounds function like rungs on the ladder, distinct stages in the lines as they unfolds (in the same way that rungs are spaced out regularly on a ladder). Perhaps the most evocative use of alliteration, though, occurs in line 15, the poem's closing line. Here, the speaker feels that they have two options going forward: they can either "fall" or "fly." The alliteration demonstrates how, though these two options bring entirely different results, the actual process in both is very similar (in that both involve leaving the "nest," as it were, and stepping out on one's own). This effect is supported by the /l/ sound consonance of these words as well.
Also note how the /f/ alliteration in the final line of the poem echoes that which appears in line 12 with "floors" and "fingertips." This former phrase is associated with the mother pinching the final bit of the tape measure as the speaker climbs up to the house's loft. As such, the alliteration of /f/ sounds once again at the end of the poem subtly suggests the strength of the relationship between mother and child; their connection is evoked via sound even as the speaker prepares to step out on their own.