This will depend on which software you use. HM Revenue and Customs free CIS Online software needs you to be online when filling in and submitting Verification Requests and Monthly Returns. However, the HMRC product can save partially completed submissions. This lets you come back to them later to add or change information.
Some 3rd party software products might let you complete forms and returns without having to stay online - you'll need to check this with the software supplier. For more information on the software products available follow the link to Construction Industry Scheme (CIS) Online - software and online forms.