Im thinking of buying a Scott Addict 20 supplied with Ultegra 52/36 rings and 11 speed 11/30 cassette. Could I change this to 52/34 rings and 11/32 cassette?If not would 52/34 and 11/30 be a compatible combination?
The combination of rings and sprockets you can use depends on the specs of the derailleurs, specifically the total capacity of both the front and rear derailleurs and the max sprocket size of the rear derailleur.
Front total capacity is just the max difference in size of the small and large chainrings, measured in teeth.
Rear total capacity is the max (difference in size small and large chainrings) + (difference in tsize small and large cassette sprockets), measured in teeth, and represents how much chain slack the derailleur can take up. You really do not want to exceed this as large-large rings sprocket combinations can damage the derailleur.
The Addict 20 comes with 105 R7000 series with a medium cage 'GS' rear derailleur. The front derailleur has 16t capacity which means you nominally can only have 52/36 or 50/34 chainring combinations.
The rear derailleur has a max sprocket size of 34t and total capacity of 39t so you could swap the 11-30 cassette for an 11-32 or 11-34.
Some people report that 52/34 is workable, and with a 11-30 or 11-32 cassette you would not exceed the TC of the rear derailleur.
Although Shimano advises against the 52/34 combination I've been using it on two different bikes* with 11-28 and 11-32 for over a year without any problems.
(*one with Ultegra Di2 and the other with 105)