I'm not sure having the sub floor venting into the porch complys with building regs, personally I would not want it to vent into my "porch" which is going to be used as a utility room rather than a porch, stale air etc. Also are you sure your not putting any heat source in ? Ok perhaps not officially but suggest you make some sort of provision to add a heat source at a later date should you so wish, say capped tails from your existing system or at least a switched spur for a electric panel heater, you then have the option.

Yes there will be an electric heater positioned on the wall on the right under all the coats, so in summer if it rains we can dry coats etc.... But central heating doesn't have to be turned on.

Not sure what else to do with the vents. There is only one on the rhs of the house, under the kitchen window that will be in the porch. There are 2 under the main window to the left.



Cheers.

If needed you will have to install an air brick on your corresponding porch wall, break through the existing wall into the under floor void and lay a duct in the new porch floor there by allowing the existing subfloor to vent. Given your proposed finished floor level in the porch it's going to be tight but looks like it might be possible, just check your levels

I take it your parking your bug in this too?

Just to add my two penneth to the discussion...

You don't need building regs for a porch that size....but should still build it to building regs.
Foundation depth can also depend on nearby foliage. For instance a hawthorn hedge just 1m high has the same impact as a tree as it is a high water consuming plant. Assuming no foliage you should at least go to the footing depth of the original house. If you dig down to that level you should hit the " bedrock" ie the hard un disturbed soil.there is no real way of guessing the required depth until you start digging and see what you find.

Vent wise you can put the ducting deeper using telescopic floor vents if it interferes with your proposed floor level...builders merchants can show you these and how they work.

Cost wise, you should estimate a builder would charge you about £1200 per meter sq of look space but this goes up as the building gets smaller. Off the top of my head I would charge about ?8k plus vat for your proposed porch


Anyway good luck!

Jon