What it measures and how to read it
The figure tells you how hard the ground may shake in a reference earthquake for that area: the higher it is, the more a new building has to comply with. NCSE-02 (Royal Decree 997/2002) gives it on its hazard map together with the contribution coefficient K, which reflects the different types of earthquake expected, and its annex 1 lists it by municipality where it is 0.04 g or more.
In English this is close to PGA, peak ground acceleration. PGA is what Spain's National Geographic Institute (IGN) publishes on its 2016 seismic hazard map, prepared for the Spanish National Annex to Eurocode 8 and calculated for a return period of 475 years.
Why it matters when buying
NCSE-02 is not compulsory for buildings of normal importance where basic seismic acceleration is below 0.04 g. Above that, earthquakes must be taken into account, and the height of brick or block masonry buildings is limited: four storeys at most between 0.08 g and 0.12 g, and two from 0.12 g upwards.
Besides the area, the year of the design matters. The requirements arrived in stages: NCSE-02 replaced NCSE-94, approved in December 1994, which in turn replaced earlier rules. An old building in a high-acceleration area may have been designed to very different rules from today's.
What the HOUSINGFAX report tells you
Check 12, «Seismicity», looks up the reference acceleration (PGA) and the K coefficient from the IGN's 2016 map at the building's location, gives you the hazard level in words (low, moderate or high) and crosses it with the year of construction to tell you which code it was most likely designed under.
Look through the sample report and check your home for free.
What this figure does not tell you
It is a hazard calculated for the area: it does not account for the particular ground under the building or how its structure responds, nor does it say whether it was strengthened later. The report sums up what the official sources say and does not replace an on-site technical inspection of the building.
Short answers
What is the difference between basic seismic acceleration and PGA?
Both measure ground acceleration as a fraction of g. Basic seismic acceleration is the one in the NCSE-02 code and its map; the PGA published by the IGN comes from the 2016 map for Eurocode 8, with a 475-year return period.
What is the basic seismic acceleration of my town?
Annex 1 of NCSE-02 lists it by municipality where it is 0.04 g or more; if your town is not listed, it is below that. The IGN viewer gives you the PGA at any point.
Is it dangerous to live in a 0.23 g area?
It is among the highest in Spain, but the risk depends above all on how the building was built. A building designed under NCSE-02 includes its requirements; a much older one may have been designed without them.
Does the report tell me whether the building will withstand an earthquake?
No. It tells you the hazard of the area and the likely design code. To know how the structure would respond you need a professional.
Official sources you can consult
The links let you review the primary source. Their inclusion does not widen their purpose or turn a piece of context into a diagnosis.