All address fields on any return submitted to HMRC, whether it is an SA100, SA800, SA900 or CT600 have restrictions on the number of characters per line of an address. These are as follows:
 

Address line 1 – maximum of 28 characters including spaces
Address line 2 - maximum of 28 characters including spaces
Address line 3 - maximum of 18 characters including spaces
Address line 4 - maximum of 18 characters including spaces
Postcode – maximum of 8 characters including spaces