The cheaper options tend to have various problems, including:
Legal issues – planning laws restrict what can be built and/or occupied in different localities. These can change from place to place and time to time. Typical requirements include permits, inspections during construction, and compliance with various regulations. It may be cheaper and more sustainable to establish housing in a different local government area. Know the restrictions and costs before settling on where to live.
Construction materials – granny flats may be built in the backyard of an established family member’s property, but materials can be pricey or difficult to source. Sustainable and less expensive options include reclaimed timber, recycled steel, strawbales, mudbrick, and earthbags. Relatively cheaper metal sheds are sometimes converted for living accommodation. While this may be permitted in some jurisdictions it might not be elsewhere. The same applies to earth buildings.
Services – access to various services may be a problem in some places. For new builds, the further the house is from power and water lines, the more expensive it is to connect to them. But you could go off-grid and become more self-sufficient. Also, consider how far you must travel for food and other supplies, or to a workplace. You may cancel out self-sufficiency gains through over reliance on transport.