Unfortunately it's not simply a job title you're looking for. It's more a demonstrable competence in navigating the two sets of rules and advising you on how best to apply them to your individual situation. This could mean an architect, an architectural technician, or a designer.
All the knowledge is on this board though, so you could take the DIY approach if you'd prefer to invest time in solving the problem rather than money? That would mean doing some background reading, drawing/posting specific plans, etc.
I have no doubt that the double storey rear extension is permitted development and therefore a certificate of lawfulness is all that is required.
The double storey extension will require a pitched roof addition which will eat into my 50 cubic feet allowance and therefore a householder application will be required. The loft I would like to do would be a hip to gable on both sides of the roof with a dormer along the full width which my neighbour has done but which I am told the council does not like
I have had one planner mention that I should do the double storey first before applying for a loft conversion hip to gable conversion. This sounds crazy
So I d like to know how to deal with the second part about householder application
I'm not certain what the certificate would be for - although conveyancing solicitors seem to be remarkably fond of them.
I really think you need someone (preferably local) to advise you. As others have suggested, look for recent similar applications (two storey extensions as well as lofts), and contact the architects/technicians concerned.
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