cost between the two, however in general an earth shelter is much more expensive. A small "underground home" built in the style of Oehler, as only a simple, non-waterproofed hole in ground with simple planks and a plastic sheet to a wall, despite using free labour (ignoring the opportunity costs of not having time to go to work) and recycled materials, e.g. windows and planks for the wall, is estimated to be 30% cheaper to construct when compared to paying a contractor to build a regular house using new construction materials at standard prices, although Oehler's design will quickly rot and collapse after a few years. A particular factor that strongly influences the cost of an earth shelter is the amount of earth that covers it. The more earth covering the structure, the greater the expense is needed in having a structure capable of withstanding the load (see also: Roof). Another important cost factor that tends to be unique to earth shelters is site excavation and backfilling. The amount of waterproofing is also more costly. On the other hand, earth shelters should have lower maintenance costs since they are mostly covered with little exposed exterior. Many financial institutions outright exclude financing on earth-sheltered homes, or require this type of property to be common for the area. Design complexity Overall it is more technically challenging to design an earth shelter compared to a regular home. Because of the unorthodox design and construction of earth-sheltered homes, local building codes and ordinances may need to be researched and/or navigated. Many construction companies have limited or no experience with earth-sheltered construction, potentially compromising the physical construction of even the best designs. The specific architecture of earth houses usually leads to non-righted, round-shaped walls, which can cause problems concerning the interior decoration, especially regarding furniture and large paintings. Repairs to the walls are very difficult to service; and may require re-evaluating and rebuilding the house from scratch. An earth shelter cannot be enlarged with an extra room - this will require breaking the waterproofing sealant cladding the concrete walls of the building. Sustainability In "green building", four "lifetime" phases of a building are described, namely material sources, construction, in use, and deconstruction (life-cycle assessment). Terms carbon zero and negative carbon buildings refer to the net greenhouse gas emissions over these four phases. Questions therefore arise as to whether certain structures are truly environmentally friendly. For