John C Daley

In 1885, a farmer in Norway came up with a very different way to get heavy loads of hay into the second floor of his barn.
The farmer was Lars Petter Olsen Valldal, and the problem was getting heavy loads of hay into the loft without making horses and wagons climb a steep hill to get there.
His answer was to build this massive stone ramp that curves around the barn, slowly climbing until it reaches the second floor.

water-conserving and water-harvesting landscapes system from a car park

Cathi, rainwater retention is not an permaculture exclusive  topic.
Just present it for what it is, rainwater retention.

From https://sirewall.com/the-sirewall-system/
SIREWALL stands for Structural Insulated Rammed Earth wall. It is a complete wall system used in modern rammed earth buildings that are energy efficient, sustainable and as strong as concrete

How will you overcome any wood rot on the logs in the ground?
Are logs that big hard to find?

What does this have to do with roof overhangs and rain?

Sometimes, knowing some side facts about a problem, IE wetness inside a home, on a wall can make it easier to understand.
Being aware of the difference between  moisture, vapour, wetness, dampness  sometimes makes it easier.
Does this help?

That looks good.
Plenty of opportunities to learn on that project.
Underground tanks are hard to get right, could you live with a big above ground tank?
5000 litres.
Did you say you are on a townwater supply.
Where are you building?

Now I understand the issue.

if the lime plaster keeps the inside of the wall from getting wet, why are overhangs so encouraged?

Wetness and moisture are two different things
Simply put, water has a few forms;
- wetness may flow out of a pipe
- moisture may be floating in the atmosphere as a gas.
- you can feel and see wetness as a liquid
- moisture being in the form of a gas cannot be felt or even seen, except as fog, but it is everywhere.

All liquids are fluids, but not all fluids are liquid
A liquid will take the shape of its container but keeps a fixed, definite volume.
Glass while solid, is also a fluid in that it will slowly move downwards and thicken !!

So overhangs prevent erosion by the water running down the wall, it also gives some protection from driving rain.
AS A FOOT NOTE
Nordic communities and I think early Irish, who may have has nordic influence, built stone homes with each stone sloping to the outside,
this preventing driving rain entering the building.