I noticed a cottage for sale which has no septic tank and no mains. As
it stands at the moment it's not possible to get a mortgage on the
cottage due to this.

Apparently the site is too small for a septic tank to be placed in the
garden, as the garden is too small and the tank has to be placed a
certain distance from the house.

I was wondering if anyone knew any alternatives to septic tanks which
might be permitted to be placed in a garden.

It's a long shot, but just thought I'd ask...!