The map below shows how Downingtown physically grew in size from its original 1859 boundaries to today. Most of the change occurred in a 15-year period between 1946 and 1961 when parcels adjacent to the Borough were annexed from both Caln and East Caln Townships – the main reason being public water and sewer access.

When public water and sewers became available in Downingtown, a policy was developed that the Borough-owned utility lines would not extend into neighboring municipalities. This generally wasn't an issue until after World War II when land adjacent to the Borough began to be developed and public sewer and water service was a much better option than wells and septic tanks. All that was required was a simple majority of property owners in a particular area to petition Downingtown’s Borough Council. If the petition was approved, Downingtown would then go through the process of annexing the land.

Ten separate areas have been annexed into Downingtown, which has almost doubled the size of the Borough's original land area – and it could have happened several more times. In 1963, the Tabas brothers wanted their Downingtown Motor Inn (where Wegmans is today) as well as their property where the old Downingtown Farmers Market stood (where Home Depot is today) to become part of Downingtown but for various reasons this petition didn’t go through. And when Whitford Country Club was being constructed in 1956, the Club’s board approached the Borough about annexation in order to get water and sewer service. However, because none of their land was adjacent to the Borough this petition was turned down.

Besides water and sewer service, property owners requesting to become part of Downingtown cited other reasons including better police protection, quality schools, and even streetlights.
Since Pennsylvania laws were being followed there was little Caln or East Caln could do as their own townships physically shrank along with some of their tax base. In 1952, however, Caln Township voters elected to become a Pennsylvania First Class Township which would result in any annexation by Downingtown (or even Coatesville on their west end) to be approved by all Township voters, not just the property owners in the area proposed for annexation. This became effective on January 1, 1953, so Downingtown hurried up the process and annexed three different areas of Caln before the end of 1952!

The last and largest annexation occurred in 1961 when 245 acres were annexed from East Caln. This was the result of the petition of several large industries along Boot Road including Pepperidge Farm, Gindy Manufacturing and Downingtown Paper Box Company.