You are not getting paid to carry risk introduced at a late stage, however well-intentioned.
- The building is now wider by 350mm on all elevations.
- The front and possibly the rear elevations are probably out of alignment with the adjoining property, assuming they lined up on the permission.
- All of the elevational details will change, especially the opes and particularly the cills, eaves, locations of gullies, downpipes etc.
- If you have already done a Bill of Quantities, there will be additional lengths for every pipe penetration.
- The side passage may now be impassable, since 165mm is not a minor amount by which to diminish a narrow passageway.
- If the distance to the boundary from windows is less than 1.0M there may be a fire implication and there may be implications for the later installation, opening up or variation of opes or glazing which could be dealt with under the exempted development regulations.
IMO you should seek a Section 5 Declaration (€80 and four weeks, last time I checked) that the change constitutes exempted development.
Just to underline the seriousness of how the local authority is taking this in relation to upgrading existing buildings, Dun Laoghaire-Rathdown Co Co now require drawings showing the proposed changes and will not give a response over the phone.
I expect a recent grant will attract the same diligence.
You're a designer, so design your way out of the problem.
You could possibly source an insulation with a higher thermal resistance than that proposed, this allowing a greater retention of heat for lesser depth of insulation.
BTW if your wall is blockwork (100mm block/ 100mm cavity/ 100mm block) and you're upping the size of the structural cavity from 100mm you need to be totally clear on what you're doing. Above 100mm I always ask for an engineers comments.
You may be advised to use a different form of construction.
Before you commit to anything either before a BER check on the cost/benefits. Insulation is often the best way to go, but the structure and detailing must be balanced. There is no point increasing the cavity and then not properly sealing the insulation to the inner leaf. There is equally no point in building in two 100mm leaves with infill insulation when a 215mm solid leaf with external weathering and insulation might over a significant cost reduction for the same level of insulation.
As always, with this depth of insulation you really need to nail your transition/junction details and do a pressure test.
And don't forget your renewables. No point doing all this insulating and still using electricity to heat the place!
Best of luck with it and you might let us know how you get on.