Register an additional company (new sociedad) in Registro Mercantil and AEAT

What you need to have ready

1. Cl@ve or certificado digital (as representative or with a power of attorney) for AEAT and Registradores.
2. A clear name for the new company (and 1–2 alternatives in case the first is taken).
3. A notary appointment (sorry, nobody avoids this). Bring DNI/NIE and partners/administrator details.

Documentation

• Certificación negativa de denominación from the Registro Mercantil Central.
• Escritura de constitución with estatutos and appointment of administrator.
• Certificado bancario of the capital social (if you don’t opt for SL de 1€ and the notary requires it).
• Modelo 600 (ITPAJD exempt) filed in your CCAA, with receipt.
• Inscripción en el Registro Mercantil (nota simple or asiento) for the definitive NIF.
• Modelo 036 (declaración censal) for provisional/definitive NIF and registration of obligations.

Step by step

1. Reserve the name: go to https://www.rmc.es and request the certificación negativa. The notary won’t sign without this.
2. Sign the escritura: bring the certificación, estatutos and details. Ask the notary for envío telemático to the Registro Mercantil and, if possible, to handle the Modelo 600.
3. File Modelo 600 (exempt) at the autonomous tax office that corresponds to you. Without the receipt, the Registro will not inscribe.
4. Register at the Registro Mercantil: if the notary didn’t do it, upload it at https://sede.registradores.org/site/mercantil. Pay fees and wait for the calificación.
5. Request provisional NIF: at AEAT (path: Sede > Empresas y profesionales > Censos, NIF y domicilio fiscal) file Modelo 036 attaching the escritura. This lets you start some procedures.
6. Exchange to definitive NIF and complete alta censal: when you have the inscripción, submit another Modelo 036 to switch to definitive and mark IVA/withholdings/IAE. Don’t start invoicing without this.
7. Get the representative digital certificate at the FNMT: https://www.sede.fnmt.gob.es/certificados/certificado-de-representante/representante-para-administradores-unicos-y-solidarios to operate with AEAT and other agencies.

Time-savers

• Have the notary teletransmit the escritura and the Modelo 600: you avoid queues and scans.
• Attach everything correctly in Modelo 036 (escritura and, when available, inscripción). You avoid silly requests and delays.
• Prepare IAE epígrafes and IVA/withholding choices before filling the 036. Don’t improvise on the form.
• Use a representative’s digital certificate; Cl@ve works, but for sociedades the certificado is usually smoother.

Common mistakes that will block the process

1. Forgetting the Modelo 600: it’s exempt, yes, but if you don’t file it, there is no inscripción.
2. Letting the name reservation expire: the certificación is not forever. If it expires, you start over.
3. Keeping the provisional NIF without exchanging it: AEAT can hit you with 200€ for not updating the census on time.
4. Choosing a name “too similar” to another sociedad: they’ll reject it and you lose weeks.
5. Registradores site: it can give you trouble. If it stalls, call the notary to resend telematically.