Experts said this was a sign the country intends to increase production of weapons-grade uranium by as much as 25 per cent.
A report from Middlebury Institute of International Studies at Monterey said the photos taken by Maxar showed construction in a plot adjoining the Yongbyon plant.
An image taken on 1 September showed an excavator on a site where trees had been cleared and the ground had been prepared for construction, the report said.
A second image, taken on 14 September, showed foundation work, a new wall enclosing the area and an access point made by removing panels from the side of the existing enrichment building.
The new area is approximately 1,000 sq m (10,760 sq ft), enough space to house 1,000 additional centrifuges to the 4,000 currently at the site, Jeffrey Lewis, co-author of the report, said.
Yongbyon also has capacity to produce weapons-grade plutonium and satellite photos from last month showed signs production had resumed.
At a 2019 summit with Donald Trump, Kim Jong un offered to dismantle the complex in exchange for major sanctions relief, but the Americans rejected what they saw as a limited step in denuclearisation.
Some US and South Korean experts speculate North Korea is covertly running at least one additional uranium-enrichment plan.
North Korea this week fired both cruise and ballistic misslies into the sea, drawing condemnation from Japan.
Experts said the tests reflected a push to boost North Korea's weapons arsenal amid a stalemate in nuclear diplomacy between Pyongyang and Washington.
Additional reporting by Associated Press