John Kelly wrote:
My extension was built and I'm about to start laying topsoil and grass
and generally finishing off. I'm a little unsure of how high I can go
with the topsoil. Obviously I can't go above the damp proof course and
that's too high up anyway but I'd like to go about 2 bricks below it as
that's what it is around the rest of the house. However there are gaps
between some of the bricks that low down - it's about 5 bricks between
gaps. Can I go up past this level or are the gaps there to let air in?

Thanks
--
John Kelly

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Blimey. What do you mean by 'gaps'

There should be no gaps blow ground, and its normal to let in proper
rodent proof vents if you need ventilation.