'Skyscraper' was an Allied unrealised plan for an amphibious assault across the English Channel onto the coast of German-occupied northern France (spring 1943).
'Skyscraper' was the last such plan prepared by the Combined Commanders' staff before the formation of the COSSAC organisation headed by Lieutenant General Sir Frederick Morgan, and as such provided one of the starting points for the planning of 'Overlord'. The concept of this plan was to allow Allied planners to break away from the overly complex developments of 'Round-up', and also to permit a more realistic assessment of the amphibious capability required for major cross-Channel operations.
'Skyscraper' proposed two simultaneous Allied landings in northern France (one on the Cotentin peninsula leading to the capture of Cherbourg, and the other on the Normandy coast leading to the capture of Caen) by four assault divisions, in preparation for a follow-up force of six divisions. Support for 'Skyscraper' was to be provided by four airborne divisions (dropped and air-landed south of the amphibious landings to interfere with the forward movement of German reserves) and 18 commando units (for special assault missions).
'Skyscraper' proceeded little further than this outline for the landings, but was based on the premise that the Allied forces would then advance basically to the east and north-east toward the line of the Seine river (though it was also appreciated that extra port facilities might be gained by the capture of Le Havre, possibly though the commitment of another amphibious landing) as the first step toward the capture of Antwerp so that major Allied forces could be built up in the region between the Pas de Calais and Antwerp for a narrow-front thrust into northern Germany.
The 'Skyscraper' planning was typified by a steep learning curve about the primary problems associated with an assault crossing of the English Channel, most especially in the number of landing ships and craft which would be required for the assault and follow-on forces and then their supply as the advance from the beach-heads began. A force of 10 divisions came to be seen as the absolute minimum to be transported to handle the current number of German formations in the west, and in the event that the Allies could not prevent additional German formations from being redeployed to France, the invasion fleet would have to be enlarged for the movement of other divisions, and two more divisions had to be made available for coast defence.
'Skyscraper' was significant in highlighting and then paving the way to the unravelling of the high demands associated with troop numbers, matériel availability, timings and costs, all of which had been among the factors which made a significant contribution to the termination of the 'Round-up' planning. The planners also pushed for a quick decision about the launch of 'Skyscraper' to ensure that the Germans were afforded the minimum opportunity to increase their strength and/or improve their defensive posture in the west, but in fact the longer a decision was delayed and the planning continued, the clearer it became that the Allies were not yet ready for an invasion of France and that the operation’s objectives were too ambitious.
The termination of the 'Skyscraper' plan thus paved the way toward the emergence of the 'COSSAC' plan and thus 'Overlord'.