While going through some articles and videos, I chanced upon a video on LCY and was surprised to see that, unlike in Dortmund, LCY has aircraft parked diagonally. Does this increase the number of bays that the airport can have in that little space, or is there any other reason other than the one mentioned here?
The main reason for the diagonal parking is so that the airplanes can taxi up to and away from the stands without tugs. LCY has a lot of airplane movements for a small space so the parking is designed around getting airplanes in and out as quickly as possible: airplanes taxi in and park diagonally away from the terminal building so when they are ready to leave they power straight out to the taxiway. Note this is only possible because they don't have any jetways, passengers walk to and from the airplanes.
The advantage of diagonal parking at LCY is that it requires a narrower portion of the tarmac thereby allowing for adequate area for the width of the taxi-track.
The terminal charts Ground Movement instructions emphasize that final parking and taxiing out can only be done with guidance from Marshaller - this indicates that standard clearances for wingtips and other extremities may not always be available, i.e. 'pilot judgement' alone, from the cockpit, is not enough to ensure safe maneuvering.
Her are some extracts from the eAIP for London City Airport:
Pilots are requested to use minimum power when manoeuvring on and off parking stands . . . .
Parking: Pilots should self-manoeuvre their aircraft on to the appropriate stand lead-in line (as directed by ATC) and approach the stand as closely as possible. Direction to the final parking position will be provided by marshallers. Under no circumstances may aircraft enter a stand without guidance from marshallers.
Under no circumstances may aircraft self park without guidance from marshallers’.
GA Apron: Pilots will be directed by ATC to taxi to the Jet Centre. Pilots must then follow the marshaller’s instructions for parking.
To ensure adequate wing tip clearance is maintained from adjacent stands, it is imperative that pilots follow the lead-out markings on all stands.
If the aircraft is not adequately positioned on stand the pilot should proceed as directed by ATC. Prior to undertaking the manoeuvre, in order to re-position onto stand, the pilot must request permission from ATC to enter the taxilane and advise ATC if the aircraft is unable to follow the stand lead-off line. Only once permission has been granted by ATC shall the pilot commence the movement into the taxilane.
Just shows that space is really tight. So to complete the answer to your question, sometimes the need for a certain disposition and use of the area is not just for maximising the number of parking slots, but it could have been a larger picture imperative - done to enable the implementation of the project at all in the limited area and disposition of usable land between the main road and river.