" it is usually wise to consult the planning department before submitting plans. OP now says she has submitted plans and is using an architect."
Most authorities now charge for pre-application consultation, so much of the advantage of doing this has now gone on simple projects.
"New houses often have no Permitted Development rights, hence would need pp for loft conversion and dormer."
That's not true, and would actually be against national Planning Policy if they did. Blanket removal of PD rights is contrary to the six tests set out in Circular 11/95. It should not happen, and if anyone has had the misfortune to have had an incompetent Planning Officer impose such a condition upon them, contact me - it's usually a simple process to get it removed.
OP: To answer your specific question on who decides, on minor proposals like this it will probably be the individual Planning Officer (albeit with their decision being signed off by a senior member of the team) at what is called 'Officer Delegated' level.
More complex or contentious applications are referred to the Planning Committee, made up of local Councillors. Whether something is referred to the Committee or not should depend on what is called the 'Scheme of Officer Delegation', which is a written document that each Council has (and they vary slightly from Authority to Authority) that sets out what circumstances and decisions can be dealt with by paid Officers of the Council, and which need to be referred to elected members.
If your neighbour is genuinely a Planner, then he may very well be able to influence the decision... even if he works for a different Authority, Planners tend to move in very small circles and he will doubtless know individuals involved. It would be unprofessional of him to do so, of course, but equally it would be very naive to think that it doesn't happen.
If the initial decision goes against you, then there is the possibility of an Appeal, which is handled by the Appeals Inspectorate, based in Cardiff, who are not going to be influenced by local friendships - but there would be additional professional costs involved with this (probably circa £750-£1,000 for a 'written representations' appeal process).
There are standard methodologies available to calculate whether the impact of development is acceptable. Your Architect should know about these, if he or she is competent: the document that sets out these methodologies, and which is accepted by all Planning Authorities in the UK, is 'Site Planning for Daylight and Sunlight', published by the Building Research Establishment.