+1 what Sconhome has said and I note teh following also; -
If its entirely to "the back and rear" and the house has not be extended previously, you may be eligible to claim that this is exempted development.
For the avoidance of doubt you and/or your architect should be talking to your planning officer with your design drawings of the proposed development, showing him anything else that has been done to the house.
There seems to be a growing reluctance by the local authority to talk with non-archtiects about design matters, possibly because of the influence of the Building Control Act 2007 and Registration.
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Even if its exempted, I would personally advise client to seek a Section 5 Declaration to this effect from the local authority.
It can give comfort if anyone decides to try and stop you legally or by writing to the local authority complaining about the work.
If you do need to go for planning don't worry about it - any saving by avoiding planning will be relatively minor in terms of money.
The planning fees are minimal and the design drawings will already be done.
You can use the 14 week gap between planning lodgement and grant of permission after signing off on the design drawings for production of the tender drawings and sourcing materials and looking at the work of prospective builders.
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Do not, in the name of saving money, fail to get some tenders drawings and at least an outline specification for the work, listing everything that needs to be done.
Without at least a good set of tender drawings and specification you are leaving yourself wide open to being fleeced with "extras".
You also need to enter into a contract and I recommend the RIAI forms as offering the best balance all round.
Your architect can advise you on which form to use.
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If you do go for planning, then yes, you need to agree the design before lodging with both planners and neighbours and yes, you are potentially exposed to an appeal,.
These are the normal processes and risks associated with any significant development.
Personally I prefer to get permission
or a Section 5 declaration that the works are exempted from the local authority.
That way when it comes it comes to the eventual sell-on and the certification, there is a surety in the buyers mind that the local authority were fully aware of the development and its status is assured.
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Clients don't always listen of course and this had led to one upset where my advices were not taken and the Section 5 Declaration backed me up.
Telling a client "I told you so" is never a productive situation, especially where they are business people but not commercial developers.
Business people tend to be insulated from people who tell them "no" or "its not permitted", but with local authorities this is exactly what gets said on occasion.
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And no, the solution is not for the architect to write a cert contradicting the local authority, whose job it it to enforce planning law.
The next step would be to apply for retention and hope damage has not been done to their professional relationship or the clients previous good name.
Local authorities are made up of personalities like any other entity and they can get ticked off with people who intentionally breach planning law.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.