"The inventory is being carried out," a source close to the investigation told AFP, without giving further details of the weapons found.
The dawn raid - a joint operation between counter-terrorism and intelligence agencies, and local police - focused on the garage of the opulent villa in the seaside resort.
The couple who own the house were arrested on suspicion of ties to Eta, according to the same source.
Eta is accused of killing 829 people in a four-decade campaign for the independence of the Basque region that straddles the French and Spanish border.
It announced the end of its armed campaign in October 2011, but has refused to hand over its weapons or dissolve the organisation, as demanded by governments in both countries.
It began a unilateral disarmament programme in early 2014, in cooperation with the International Verification Commission, an independent group of experts from various countries that has not been officially recognised by the Spanish government.
Earlier this month, Eta once again rejected a disarmament plan put forward by the Spanish Basque regional government.