London’s Heathrow and Gatwick airports have been identified as possible sites for new runways to boost airport capacity around London.

In an interim report, published Dec. 17, the Airports Commission put on ice options for a new-build airport in the Thames Estuary due to the high cost — beginning at £112 billion ($182 billion) — and because it presented “many challenges and uncertainties.”

Despite describing Heathrow as full in 2010 and Gatwick due to be at full capacity by 2020, the commission points out that a new runway would need to come into operation by 2030. At Heathrow, commission officials have identified two options. One is the construction of a new 3,500-meter runway to the airport’s northwest, suggested by Heathrow’s owners last July. An independent option, proposed by the Heathrow Hub group, would lengthen the airport’s northern runway to at least 6,000 meters, enabling it to be operated as two separate runways: one for departures and one for arrivals.

At Gatwick, the commission says its analysis will study options for a second runway at a location south of the current runway that would allow for independent operation, one as an arrival runway and the other for departures.

Heathrow’s owners said during the summer that either of the third-runway options it presented could deliver new capacity by 2025-9 for around £17 billion, less than a quarter of the expected price tag of a new airport.

The commission also forecasts the requirement for a second additional runway in England’s southeast by 2050.

While many of the Estuary airport options promoted by London Mayor Boris Johnson weren’t shortlisted, commission officials have said they will study the most viable: a new-build airport on the Isle of Grain. But the report points out that any of these airport ideas would require “substantial” surface access infrastructure, with “potential cost, deliverability and environmental challenges of [their] own.”

“The overall balance of economic impacts would be uncertain — particularly as an Estuary airport would require the closure of Heathrow for commercial reasons and London City for airspace reasons,” the report adds.

Plans for a second runway at Stansted were also not shortlisted. But the commission says the airport is a “plausible option” for any second additional runway in the 2040s.

The commission will now firm up studies and development work for the final report, to be delivered in 2015 after the next general election.

The commission is also pushing for a series of short-term solutions to capacity, including the optimization of U.K. airspace, using enhanced en route traffic management systems, and performance-based navigation. It also calls for conducting trials at Heathrow to smooth early morning arrival schedules to minimize delays and provide more predictable relief from noise and pollution for local communities. The report also urges improvements in surface access to airports, including better rail links. But while officials say such measures are “worthwhile on their own terms … none of them can provide a long-term solution to the U.K.’s airport capacity problem.”