Crawling, each area has a 'method' of deciding how decisions are made. That is not the same thing as a 'rule' because the only 'rule' is the SEN Code of Practice, which tells Local Authorities what they must do and what they should do.
Statutory Assessment Request
If you have put in a request for a Statutory Assessment, then the Local Authority must inform you that they have received such a request and the contents of that notice are defined in SEN CoP 7:16 (page 77). The Local Authority must tell you within 6 weeks, whether they will be carrying out a Statutory Assessment (SEN CoP 7:26, p. 79).
Statutory Assessment
If your DD does have a Statutory Assessment, the LA must seek the following advice in writing:
Parental, educational, medical, psychological, social services (SEN CoP 7:82, p91).
Now, that doesn't mean reams of information, necessarily. For example, social services may just fax back a piece of paper saying 'not known to Social Services', which is fine.
Your evidence may be requested using a form with little tiny boxes for answers. That doesn't mean that you have to use that form. I used the same headings as the form gave, but typed my own report. It was around 30 pages long...I can't quite remember now. The form they gave had two sides 
You will have an appointment with a Paediatrician, who checks your child over and makes comment on medical history and medical needs. For example, DD1's paed said it would be helpful to have step markers and a wet room available.
The Ed Psych should assess and give recommendations.
OT and Physio may need to submit reports, as will anyone else involved, such as Portage.
Then, the LA must decide whether to issue a Statement within 10 weeks.