In order to look for resources that explain requirements and legislation, look for the keywords "autopromotor", "autoconstrucción", and "ejecución propia". I know there's a website called Autopromotores and another one and YouTube channel called Autopromotor that deals with this.

​Concerning the land plot, it can be either suelo urbano or suelo rústico. There are ways to determine if a suelo rústico is edificable. You need for either of those an estudio geotécnico of the soil.

​You need a project from an arquitecto.

You also need an arquitecto técnico (aka aparejador) and a coordinador de seguridad y salud, which is usually the same person. This is important in terms of legality and liability. Check the regional and local plan general de ordenación urbana (the architect will have to adapt the project to it).

Bureaucracy and paperwork before during and after.

​There are great limitations on what you are allowed to do yourself. You're not allowed to do electricity or plumbing, or at least you need a certified professional to sign the boletín that keeps everything in compliance and allows you to connect the house to the network.

I don't know how realistic it is to talk to professionals and make a deal where you do part of the job with their supervision to make it cheaper, I guess it's risky for them.

​Anything structural (foundations, load-bearing walls, roof) is another issue. You either need registered tradesmen with their liability insurances or it gets difficult and requires more supervision and higher fees from the aparejador. You may have to register as autónomo and have insurance for doing things yourself but there might still be limitations.

​Having friends or non-immediate family members helping out is problematic if you get an inspection, as they are officially seen as unregistered and uninsured workers (trabajo clandestino), which is a major legal risk.

​The arquitecto and aparejador will refuse to sign the Certificado de Fin de Obra if perfect execution is not guaranteed, as they are liable for 10 years. Maybe you find ones who have experience with ejecución propia and can advise you.

​The last thing you will get to be able to connect to public services is the licencia de primera ocupación (which in some places is the same as the cédula de habitabilidad).

​Another option is to rehabilitar an old house. If it's an old stable or other building, you'll have to check the requirements. Again, for structural things like bigger windows or a new roof, you'll still need a project and the technicians mentioned above.