wrote:
I need to have full plans for my extension, due to building near a
public sewer. I also want to be able to fully budget before starting,
and dont want hassles with revising plans etc.

Not quite clear from your post but it looks like you intend to do drawing
work yourself.
Having just gone through this excercise, I would recommend the following:
Submit outline of your plans to council with "Do I need planning
Permission?" form.
If "Yes" then put in formal application.
If "No" then keep their written response as proof of this decision.

Produce drawings in as much detail as possible, then find trust worthy
builder who you would like to do the job, to fill in the details you don't
understand.
It may be worth offering a consultancy fee - say £50 for their guidance that
will get them interested in doing the job proper maybe.
This process saves paying £200-£300 for others to do the drawing work
(incorrectly in my case) and the to-and-fro getting changes made etc.

Once you are happy that you have covered the majority of details, submit for
building regs approval, highlighting issues you are not clear on.
They will advise the best way to resolve these. Building close to or over a
drain/sewer does require additional works for foundations but usually
depends how close.

HTH

Phil