If you've got a 280 plate, then using a 600 base out from the wall and the baseplate tight to the wall will mean a difference between centre of loading and centre of base of 300-140 = 160mm . This is greater than L/6, meaning that the whole of the underside of the base out from the wall is not loaded (which is another story, trust me on this!), which in turn means that the pressure at the the leading edge of the base is quite high - and might be higher than the bearing capacity of the soil. However, as there is already a foundation and wall in front of it, the base would have to push those out of the way to rotate and cause a bearing failure.
But...let's assume that the BCO won't accept that, having been educated only in the barest essentials of loading and foundation design. And, anyway, this base would need some designed rebar in it, running at right angles to the wall and secondary rebar running parallel with the wall, which all starts to get a bit silly.
The base you are putting in is 600x600 = 360000mm^2. I'm assuming that this is ok for the load being applied to it - unless you're on sloppy crap, I would think that it's fine. Now, if you were to change this to a rectangular affair, let's say 300 wide out from the existing wall, then the length would need to be 1200 long to get the same area. THe dimension from plate edge to foundation edge would be (1200-280)/2 = 460, so as long as your base is greater than 460mm depth, no reinforcing is required, as the load will spread along the length of this base. Looking across the other way, the centre of the post would still be at 140 off the outer edge of the foundation, ie 10mm off the centreline of the base - in other words, next to no eccentricity and certainly way less than L/6.
If you want to use a 450 wide base out from the wall, the same maffs applies. Base length needed is 800 and the minimum depth is 260mm. Eccentricity across the short dimension will be 55mm, not worth worrying about, as it's still less than L/6.
Therefore, whichever rectangular base you decide to use, there is no need to worry about reinforcing it and you can save those offcuts for something else.
Personal preference? 800x450. Don't forget to go down to the depth of the existing foundation as a minimum, of course, with a min depth of 260mm of footing.
Re the other post, no that was me: the mods (well,
a mod) were being particularly stupid the other day, so I wrote delete to save them the hassle of deleting my post, as anything I was posting seemed to arouse their ire. I'll get around to answering it properly
.