a porch is not b.c. notifiable so regulations are not imposed.
however, as with any foundations, wisdom suggests you dig down through fertile material usually 600mm plus, until you reach a firm virgin substrate.
if you are still fishing out old tie wires, beer cans or ancient pop bottles - dig deeper or forget the project.
225mm thick concrete is suitable for almost all domestic extensions so it will 'eat' a porch.
by the way, should you remove the external door separating the porch from habitable dwelling, you render the porch 'habitable' thus engaging b.c. approval.
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