You're permitted to build an extension of up to 40 square metres to a house. Does that apply to a new house? Can someone build a house and immediately add on 40 sqm?
And conversely, have I a better chance of getting planning, if I go in for a smaller house, and add the extension onto it afterwards?
Once the new house is finished completely you can add the 40 m2 subject to the usual conditions. If you build the new house smaller you will need planning for that.
once the original house exists, then it can be extended immediately.
its cannot be "extended" at the same time as building the original, as thats not technically an extension.
One of the first conditions of a granted planning permission will be that the works have to be completed as per the drawings and specifications agreed as part of planning. That means for your certifier to certify the works, the works have to be completed as per the planning (and therefore not including any exempted development works).
So you would have to build the works as per planning, have it certified, and then you can carry out additional exempted development works. This usually isn't cost-effective. You could do some of the substructure works at the same time and leave things ready to let you build the extension, but depending on the design/layout it also means completing the works as per planning including all finishes etc, and then immediately undoing sections of it.
Also important to note there are numerous other restrictions regarding exempted development other than just floor area.
I would say depending on your own time/cost limitations, your best course of action would be go for planning for what you want (including the additional 40sqm), and if that section isn't allowed or taken off as part of further information, then take it out for planning and build it as an exempted development after.
Thanks for the advice. Can it be a condition of planning that no further exempted developments can be carried out?
It can indeed but it doesn't rule out the possibility of applying for and being granted permission.
strictly speaking, the works outlined in the planning permission have to be finished and certified before any further extensions are added.
now having said that, ive seen jobs where everything was built at once and the engineer just worked around it when certifying. wouldnt exactly recommend doing it this way though as its potentially more hassle down the line