Tube stations in Watford, Croxley Green and Rickmansworth will join the rest of the capital from January, being placed in numbered zones for the first time.
Transport for London has revealed that three new zones, numbered 7, 8 and 9 will replace lettered zones from the beginning of next year.
Watford, Croxley and Rickmansworth currently fall within Zone 6A, which is often not included in travelcards issued from railway stations.
As of January the stations will be in Zone 7, with Watford High Street and Bushey being placed in Zone 8.
Ticket prices for the other stations will not change under the new zone classifications. Single fares on Oyster cards between Zone 7 and Zone 1 will be £4.50 on weekdays 7am and 7pm and £3 at other times. Fares on London Overground services from Watford Junction to Zones 1-8 will be £6.50 on weekdays 7am and 7pm and £4 at other times.