One quasi-engineering approach is to find a similar vehicle (or a similar set of components) that either exist or are fairly fully designed and use their masses as first-order estimates. So for your example, and relevant to this thread, how different is the habitable volume on your ascent stage from the habitable volume on the Altair design?
Well, that's more or less what I've been doing, but in a lo-fidelity sort of way which I wanted to improve upon. Right now? My ascent stage doesn't have
a habitable volume, it has a mass, because I was focusing on getting the propulsion to close. 8 mT dry, about 33% bigger than the Altair ascent stage (fueled? the data sheet I found from JSC doesn't say); but
, of course, this needs to do a lot more delta-V and can't offload as many life support services to the descent stage.
EDIT: Well, I have given the functional layout some thought, despite what I might have said above. In my mind, there are three functional (pressurized) areas, the habitat module on the ascent stage, a "pantry" on the payload stage, and an inflatable airlock attached to the "pantry".
The habitat module would contain all of the essential life support equipment, along with 12 person-days of life support consumables. It would serve as home to the crew during transit to and from the Moon, as well as on the Moon's surface. I imagine it as being an aluminum ISS module-type structure, although there's no reason I suppose why it couldn't be inflatable.
The "pantry" would contain the other 40 person-days of life support consumables along with any lunar surface equipment needing to be kept in a pressurized environment during L2-Moon transit (eg., the space suits, at least before use). It would serve as the storm shelter during transit from L2 to the Moon and on the Moon itself. Additionally, it might have some other surface-specific equipment, like a dust mitigation compartment. This could also be inflatable, although as before I was imagining it as hard-shelled.
The inflatable airlock would be an airlock. It would store the suits on the lunar surface, and possibly some other surface-specific equipment.
To achieve the goal of being as flexible as possible in the basic design, both the ascent stage and payload module would have spaceframes that would support the pressurized modules--the pressurized modules would contribute nothing structurally (so that it would be easy to build an all-cargo version), or in terms of other essential lander components (eg., avionics). The ascent stage would support four engines roughly the size of the AJ-10 or the LMDE in terms of thrust, and the necessary propellant and pressurization equipment in addition to the habitat. These would be located well above ground, like on the DASH concept, meaning that the lower (payload) module could have a very low ground clearance, enough to make the issue of ladders irrelevant.