How does the author build tension?

The author builds tension in several ways. 
She begins by introducing a conflict early in the story. Accordingly, riots have erupted outside the city. The perpetrators of the riots are "people of another color." The conflict between two distinct racial groups spills over into the suburbs, where burglaries are becoming the norm. 
Next, the author divulges that trusted members who are "people of another color" are the only ones employed by homeowners in the area. Gordimer fractures the subversive group further into "trusted" and "untrusted" categories. Now, the "trusted" category of housemaids and gardeners are pitted against the "untrustworthy" in their racial group. So, there are different levels of conflict in the story. 
Another way Gordimer builds tension is by using sound and visual imagery to describe the prevailing anxieties in the characters' lives. For example, homeowners in vulnerable suburban areas must live with hi-tech alarms that often increase the tension in their lives.

The alarm was often answered-it seemed-by other burglar alarms, in other houses, that had been triggered by pet cats or nibbling mice. The alarms called to one another across the gardens in shrills and bleats and wails that everyone soon became accustomed to, so that the din roused the inhabitants of the suburb no more than the croak of frogs and musical grating of cicadas' legs. 

Also, desperate homeowners are shown to resort to extreme measures to protect themselves. These extreme measures lend a claustrophobic, threatening intensity to the story. This is another way the author builds tension.

...there was the low-cost option of pieces of broken glass embedded in cement along the top of walls, there were iron grilles ending in lance-points, there were attempts at reconciling the aesthetics of prison architecture with the Spanish Villa style (spikes painted pink) and with the plaster urns of neoclassical facades (twelve-inch pikes finned like zigzags of lightning and painted pure white).