I am currently in the throws of trying to convert from an OBRAS (works) contract with Endesa to a normal Vivienda contract. It all seems very protracted and has cost us quite a lot of money.
The paperwork requirements include;
Bank Account details,
Licence of First Occupation and the absolutely essential Bolletin.
If the house currently has a meter box outside of the old type (about 9" wwide x 15" high) this will have to be replaced with an up to date legal version. The law changed in 2007 relating to these boxes, as it did for the internal boxes too.
I found this link to the Iberdrola website which explains all (in English.) Regretably Endesa have chosen not to put an English version of their website online.
It might well be that you both need a works contract initially so I would suggest you definitely talk to the Town Hall They will be able to point you in the right direction.
If you are making any structural changes to the properties you will need planning permission from them anyway. Even minor works such as changing a kitchen or bathroom, renewing plumbing or drainage need to be notified and the proper permissions obtained. When all of the work is complete the Town Hall will probably request a survey be carried out by an independent Technical Architect to make sure that not only is the house habitable but that the work carried out is in accordance with the original permissions and any existing and/or subsequent building regtulations. Whilst the work is ongoing you can expect a visit from the planning office at any time. The survey report is a pre-requisite for the Issue of the Licence of First Occupation and this in turn is a pre-requisite for a vivienda contract from the Electricity Supply Company, no matter which one you choose to use.
It may well be that there are other hoops to be jumped through
In our case a breakdown of charges incurred to date are as follows;
Legally required Modifications and improvements to the existing electrical system and issuing Bolletin 950 Euros
Independent Architect Survey Report; 250 Euros
Initial Works Licence from Town Hall; 200 Euros (This is based upon the declared value of the property ie Purchase Price)
Re connection Fee; as yet unknown
Licence of First Occupation; no charge.
Hope some of this helps!