Planning cob building for later addition

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.

Lime plaster is protective, but if a large amount of water gets on the wall, it may penetrate the plaster layer and get to the wall material and weaken the wall/plaster interface and get to clay based material, so it's wise to protect such a wall with overhangs.
Have you thought about a roof window with an interior shutter? You could keep protective overhangs (if unstablizied wall) and have direct sun heating option, easily controlled by the shutter?

John, are you saying that the lime plaster is meant to protect only against humidity and not rain?

Cristobal, I've seen that some adobe houses have a cement plaster on the lowest portion of the wall (about a meter), and a natural plaster above that. The window is an interesting idea. You're saying that the sun would cross the window, and heat the norther wall? I've thought about a skylight (I don't know if these are the same as roof windows) with an interior wooden door/shutter that can be closed in the hottest months.

Your idea made me think of a removable or retractable awning. In the months here when the sun is low enough in the sky to warm the walls, it generally doesn't rain. The awning could be extended or installed before the rainy season.

I meant skylight, but did not want to call it this way, because I associate them with something without interior shutter and even not openable.
Awning can be easy torn by winds, and in my opinion would be short lived.

John C Daley wrote: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.

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

Gutters also prevent water from running down the wall. In this climate, an overhang of 30 cm is considered conservative, and doesn't block much sun. A meter-long overhang would block a lot more sun.

I agree that an awning would be too flimsy. Maybe 'detachable overhang' would be a better term, but it sounds too finicky for me.

I like the idea of transparent overhangs. A transparent wrap-around porch is another possibility.

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?

I think the easiest way to make your idea work is to design each room as a separate structural module, but have the rooms share a wall.

For the second room, I would not try to make its centre tree-trunk beam depend on the first room's beam. Instead, both ridge beams could meet at the shared wall, with that wall designed from the beginning to support the beam ends.

That support could be a properly designed timber post, reinforced pier or another structural detail incorporated into the wall. The important part is that the roof load has a clear path down to the foundation rather than relying on cob alone around a beam pocket.

If you know where the next room will eventually go, I would also plan the doorway in that shared wall from the beginning. That way you are not later cutting a large opening into a wall that may be supporting part of the roof.

This could reduce the second room to three new walls instead of four, while still keeping the dividing wall between the rooms. I think keeping those cross walls could also be useful because a very long chain of rooms will need good lateral stability.

The oval shape may make this more difficult. One compromise might be to keep the outside ends and corners rounded, but use straight walls where one room joins the next. You would still get much of the character of a cob building without making each future addition complicated.

For the roofs, I would keep the same ridge height, pitch and orientation for each room. When another room is added, its roof can simply continue the same general roof line rather than creating valleys or complicated intersections.

I would also make the roof overhang on the future-addition end of each room easy to remove. While the room is standing alone it protects the wall, but when the next room is built that section can be altered and the new roofing joined properly.

Your plan for a long southern wall also makes sense for passive solar heating, especially with your strong winter sun. I would still avoid making the overhang unnecessarily deep on that side. A combination of correctly sized overhangs and adjustable window shading may give you more control. This guide on plantation shutters vs roller blinds (https://iseekblinds.com.au/learn/plantation-shutters-vs-roller-blinds) explains some of the differences in controlling sunlight, privacy and heat, which may give you some ideas even though your building style is quite different.

I would also be careful about choosing the room width only according to the length of the available trees. Timber species, diameter, taper, knots, roof weight and actual span all affect how much a beam can safely carry.

If it were my project, I would spend some money at the beginning getting one repeating structural detail checked locally: foundation, wall, bond beam, ridge-beam support and roof connection.

Once that detail is worked out, you could repeat essentially the same small room each dry season.

That seems to fit your original plan quite well: one manageable room at a time, simple roofs, locally available timber, shared walls between rooms and no complicated roof junctions as the house grows.