How to resize board

I designed a PCB but I forgot to put mounting holes in and there’s no room left on the PCB, what options do I have to extend the board so I can add holes at the corners, is it as simple as click and drag the rectangle out a bit?

Yes, simply make the shape on the Edge Cuts layer which details the board edge larger.

If you used rectangle as PCB shape - yes.
If you defined shape with lines - you will have little more work.

Well I actually had somebody else design the pcb but I’m trying to learn kicad and how to amend it tp add these mounting holes.

So they had arced corners, I did try and move the arc out and move the side out but I just couldn’t get them to align perfect so I decided to just delete the whole border(arced corners and straight sides) and replace with a solid rectangle.

I now find that I’m getting these following errors which point to the pins on the MCU. Every pin has this error.

[solder_mask_bridge]: Front solder mask aperture bridges items with different nets
Rule: board setup solder mask min width; error
@(223.9025 mm, 85.5200 mm): Pad 1 [Net-(U3-SEG7)] of U3 on F.Cu
@(223.2675 mm, 85.5200 mm): Pad 2 [Net-(U3-SEG6)] of U3 on F.Cu

mcu

Any ideas what’s going on?

To get your fillets back:

  1. Select the rectangle.
  2. Right click for the context menu, and Shape Modification / Fillet Lines.
  3. Enter some desired radius.

It is a combination of the pad settings and the distance to the other pads next to it. Apparently solder mask expansion is used. Actual (copper) clearance may also overlap, but the resolution of the screenshot is too low to tell. I don’t know what the pitch of this thing is, and whether solder mask fingers would fit in between. Such things also depend on the accuracy of the machines used by the PCB manufacturer. Quite often, no solder mask expansion is used at all, and this often results in the PCB manufacturer using a solder mask expansion that is just enough to clear the pads with the tolerances of their own machine. But there are no guarantees here.

The point is that these errors didn’t exist until I delete the border and replaced with a rectangular border

Someone else designed the board, you changed it: if your settings and his settings are not identical, then it’s quite possible to get these things happen.