Model magazines have been telling readers to avoid gaps under buildings for over 50 years, don't let your layout down with this simple mistake.
The first step to avoiding a gap along the base of any wall is to make sure the building is firmly fixed in position. Try to create as level an area for it to sit in as possible and then glue the model (I use UHU for this) down using a bit of weight to hold it in place. The hammer shown here has been carefully positioned as I wanted to fix the shed down, not make a hole in the roof.
Ballasting the shed area had deliberately left space for the building to sit in position. Once fixed, the ballast is brushed up to the edge of the walls, trying to keep as thin a layer as possible. The same method works if your building is sat in a field, where flock can be brushed up to the base of the walls.
The ultimate gap cover is either electrostatic grass or coarse flock powder. A puffer bottle is a perfect way to add static grass here as you can really blast it into the corners. Flock takes a little more delicacy, place clumps with a pair of tweezers as these are easier to get into the nooks and crannies than your fingers.
As you can see from the photo at the top of this page, with a bit of work, the undergrowth becomes a feature in its own right. Here we have some waste ground with a build-up of scenic material including bristles dipped in coloured flock to produce foxgloves.
All this only takes a few minutes, but the difference it makes to your model is massive. Happy bedding in!