yishuv ha'aretz: chovas ha'gavra or mitzvah in the cheftza of the land?

Yesterday I slightly misquoted the Chasam Sofer (Sukkah 36a - link to the source so you can see it for yourself). He writes that when Rabbi Yishmael opined (see Brachos 35) that Torah study may be interrupted to plant and harvest, he was speaking only of Eretz Yisrael, where the cultivation of the land is itself a mitzvah. Just as one must make time to fulfill the mitzvah of tefillin even at the expense of Torah study, so too, one must make time to fulfill the mitzvah of yishuv ha'aretz by cultivating and building the land even at the expense of Torah study. And, adds the C.S., we should not be literalists and think that planting and harvesting alone are permitted, but all activities which contribute to the development of the land are included. However, outside of Eretz Yisrael, where there is no mitzvah to contribute to the development of the world, no mitzvah of yishuv ha'aretz, Torah study takes primacy over all other activities.

My wife raised a good question regarding this chiddush and the mitzvah of yishuv ha'aretz. Assuming one is already living in Eretz Yisrael, is there a chovas hagavra to personally undertake activities for the sake of yishuv ha'aretz, or does yishuv ha'aretz simply mean that the cheftza of the land must be developed? For example, let's say a person living in Eretz Yisrael wants a new home -- does yishuv ha'aretz mean the individual is personally obligated to build that house (assuming he knows how), or does yishuv ha'aretz simply mean that there is a mitzvah for the home to get built, irrespective of who does it (Arab labor?) or how it gets done?

The chiddush of the Chasam Sofer hangs in the balance. If yishuv ha'aretz is just a chiyuv in the cheftza of the land to be developed, but there is no personal obligation to actually do the building, it cannot be compared to the mitzvah of tefillin. One cannot hire someone else or even appoint a shliach to don tefillin on one's behalf; it is the act of donning the tefillin on one's own body which the Torah demands. Is it similarly the act of building or farming which constitutes yishuv ha'aretz, or is it simply accomplishing the end result of having the land developed? If all that matters is the end result, it is hard to see why Rabbi Yishmael would allow for an interruption from Torah study when one can hire workers or accomplish that same result in some other way.

What makes this a tricky question is that yishuv ha'aretz is a multi-layered mitzvah. When we speak of yishuv ha'aretz in the sense of moving to Eretz Yisrael, the mitzvah obviously entails a chovas hagavra. You can't appoint a shliach to move to Eretz Yisrael and fulfill yishuv ha'aretz on your behalf. Yet, when we speak of yishuv ha'aretz in the sense of building and developing the land, it seems that the focus is on the end result, not the personal participation.