Add an additional activity to your company (Registro Mercantil + AEAT)

What you need to have to hand

1. Certificado digital de representante installed and working with AutoFirma.
2. Estatutos vigentes and your escritura de constitución.
3. Clear text of the new objeto (if you must expand it) and the junta agreement.
4. Epígrafes IAE exact (code and description) that you'll register.
5. Premises details (address and referencia catastral) if the activity is in a physical place.
6. IBAN if you will set up direct debits for taxes.

Documentation

• Acta o certificado de la junta approving the modificación del objeto social (if needed).
• Escritura pública of the modification granted before a notary.
• NIF de la sociedad and details of the administrador.
• List of epígrafes IAE to add.
• Lease or title for the premises (if any), with referencia catastral.
• Poderes de representación if an apoderado signs.

Step by step

1. Check the objeto social: if the new activity doesn't fit, draft the new text (no vagueness) and approve it in junta.
2. Notary: take the agreement, elevate it to público and they'll give you the escritura.
3. Registro Mercantil: file the escritura in your RM (better online at sede.registradores.org). Pay aranceles and wait for the inscripción and the BORME.
4. AEAT – Modelo 036: go to the sede, use the declaración censal. Mark “Modificación”, add the epígrafes IAE, the start date, IVA/IS regimes and the activity address. Sign and file; keep the receipt.
5. IAE – Modelo 840 (if applicable): if you are not exento, register the epígrafes with quota. If you are exento, do not file Modelo 840; the 036 with epígrafes is enough.
6. Review: adjust your VAT books, invoicing and contracts to match the new activity. If you have staff or a premises, prepare Seguridad Social and apertura de centro de trabajo procedures.

Tricks that save you time

• File the 036 first if your objeto already covers it. If not, modify the estatutos first to avoid inconsistencies.
• Bring a list of alternative epígrafes IAE in case your first pick doesn't apply; changing a 036 is quick.
• Keep AutoFirma open and updated to avoid the classic signing error on AEAT and Registradores.
• Ask the notary for a copia simple electrónica: it helps to fix RM issues without extra trips.

Common mistakes that will mess up the process

1. Picking the wrong epígrafe IAE and then not being able to apply the right IVA regime. Fix: check the list and, if unsure, pick the most specific; correct with another 036 ASAP.
2. Not modifying the objeto social when required: RM will issue a note and block other procedures (banks, licences). Modify the estatutos if there's a risk.
3. Filing the 036 late: you can get the 200 € census sanction. Don't wait until you invoice.
4. Putting a premises without referencia catastral: the 036 can ask for it and block submission. Have it ready.
5. Trying to sign with Cl@ve as a company: for legal persons use the certificado de representante. Cl@ve will error out and waste your afternoon.