An important advantage of using Perspekt is that you get professional plans backed up by live 3-D CAD models, without the overheads and general pretentiousness of an architect's office.
I have, so far, 100% record of getting Planning Consent, and would advise if I thought consent was unlikely.
Don't forget that an extension scheme can often be enhanced by some internal reworking of the existing building, using new steelwork where necessary.
Not shown are numerous smaller extension jobs. Some of these have been Permitted Developments, while in other cases a conceptual design has been tweaked to bring it within Permitted Development.