I've heard it said that, when considering the size of your cob house, go only as big as you need now because you can just extend it when the time comes. It has also been said that creating a cob extension is easy.
How easy can it be...really? You can't just bust down a wall with a sledgehammer or saw through it like a normal house. It's a solid, monolithic piece of earth you have to deal with. I can't see how it'd be too easy...but is it?
Why would you need to remove the whole wall at the extension? As long as you plan openings (windows, doors) to fit your ultimate circulation desires, ending up with thick interior walls in a few places doesn't seem like a drawback.
Also, you can chop a hole in a cob wall pretty easily with an axe or pick, and patch or reroute the cut edges.
If your cob is structural(holds the roof up all by itself) you certainly can't just knock down a wall without serious work to stop the whole dang thing coming down on you.
OTOH, if you have a timber frame(or whatever, something aside from just cob) supporting your roof, you probably can knock out just about any walls you want. Sure, it's solid and monolithic, but it's earth, not stone. You can take it apart with hand tools. And after you tear a wall apart with a mattock, the debris can go right into the mix for the new cob you need, instead of into a fire or dump like a stick-frame wall!
Easier yet, what about adding without knocking down any walls at all? Adding complete rooms would seem pretty efficient; a good sized eat-in kitchen with sleeping loft could be the entire house, to start with; rather than expanding by knocking down a wall, you could add additional rooms next to it, either by attaching to a secondary door, or by building in front of the main entrance and moving the entrance into the new room. Extra thick interior walls aren't really a problem unless you're dealing with fairly severe lot-size issues...
The main potential complications, as far as I can see, would be the roof and the foundation/drainage.
As far as the roof goes, in my area I see a lot of shitty add-ons to stick-built houses with terrible roof-lines. The original house is usually gable-roofed and single story, oriented with the roof sloping towards the street and back yard. To add space as cheaply as possible a shed-roofed expansion is stuck to the back... but to have enough headroom, the pitch must be unreasonably shallow, and the connection point is often very awkward looking as well as a potential leak spot. I've also seen the same thing with other types of roof; a pyramid hip roof added onto like this is dreadful.
Simply building a gable roof with the slopes facing the side yards would allow easy expansion to the front or rear. For a small structure I'm a big fan of a simple shed style roof, which is easy to expand in 3 directions.
Of course you could also build your initial room with some sort of roof that is not well suited to expansion as built, like a pyramid hip roof, and deal with expansion by lifting the initial roof to a height that allows proper slope of additional roof space. You could also build more roof than you need from the beginning, and enclose the covered space gradually...
'Theoretically this level of creeping Orwellian dynamics should ramp up our awareness, but what happens instead is that each alert becomes less and less effective because we're incredibly stupid.' - Jerry Holkins
As Dillon says, additions onto rooflines can be problematic, but if you are starting small with expansion plans (even fuzzy ones like "we'll add some more rooms on this side"), you can easily build your first roof so it does not have a low edge where you want to add on. Whether that is a shed, properly oriented gable, or something else is immaterial as long as it is planned to work with the future layout.
Foundation issues are also important; if you don't get good drainage to start with, an addition in the wrong place can cause major problems later. This is not something that can be described as obviously as rooflines, but you need to determine where the water is going now as well as in the future. Roof drainage water is part of this equation too. You don't want the roof gutters to end up pouring onto the uphill side of the house.
In The Hand Sculpted House written by The Cob Cottage Company out of Oregon, they talk about starting small, but having the rough outlines of a larger, more expansive home already planned. This could be as simple as a bubble drawing with vague room types intersecting. I recently completed a kitchen add-on out of cob where the owner had intentionally left door sized openings in her East and West walls. They had been sealed and insulated for ten years until this project when she tore out everything filling the door frame. In a day she had access to her new cob kitchen addition. This scenario could be done in a cob home as well, if vague ideas of where extensions might be placed are mapped out before hand as Glenn suggested. Taken a step further, the roof line could be vaguely planned and hardware to install the roof to could be built into the cob wall, waiting for a new roof extension to attach to it.
Acetylsalicylic acid is aspirin. This could be handy too:
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