I can definately help you here and flag up a few things to add to your check list of things also to watch out for.;
– Assuming you have the head height,?
– Done any survey of the existing roof structure to ensure integrity?
– Identified what type of loft conversion you are looking for to do?
– Check if it is within permitted planning
– Not a conservation area?
– Party wall agreements needed if you are in a terraced home.
An architect would be the natural step as I always think of it like the lego box, so many people start costing with just the picture on the box but you really need the instructions (building reg drawings) before you can even start to cost, establish the right ventilation, type of insulation, airflow, materials quantities and how everything fits together i.e. scaffolding, glazing e.t.c e.t.c
You will without a doubt find any builder that costs without these detailed drawings will caveat and add in more costs to cover the risk of not having the exact drawings done.
A few things to watch out for, getting the buidling physics in a loft is really important, its is very easy to end up with very energy inefficient lot conversions, very cold in winter and very hot in summer through poor insulation and poor ventilation and heating. This happens so often. More insulation means warmer in winter and cooler in summer and explore wood fibre and other natural insulations rather than glass fibre and rock wool and PIR, there is growing concern in the industry.
Also assuming a bathroom may be up there also means really exploring your water pressure too and shower and using an aerator helps to reduce water flow but feel powerful for example.
Await your response and where are you based?