Sewers tend to allow high-density building to an extent that would not be possible with septic systems. In fact, it even becomes profitable to tear down existing homes and build new housing at higher density. This is made possible because with sewers, developers no longer need to reserve land on the lot for a septic system.
We can see this trend at many places along the South Acton sewer route. For example, consider Prospect Street north of its intersection with Rt. 27. Before the sewers, there were 22 single family homes on this street. Since sewers, the number of housing units has doubled from 22 to 45. Several single family homes were torn down in order to build new structures at high density.
This is illustrated below. Here is 148 Prospect Street before sewer-enabled building:
And here is the same lot on Prospect Street today. The house above and the house next door were torn down and replaced with houses built close together:
In another example, consider 282 Main Street, where a single family home was torn down to make way for 6 units. This is the lot before construction, in 2007:
And here is the same lot after construction:
In a third example of the high density made possible by sewers, consider 446 Mass Ave (see here, page 22). A single-family home is slated to be torn down to make way for 31 new housing units, on less than an acre of land:
These high-density projects would not have been viable without sewers.