IBI calculations

I believe that my IBI has been wrongly calculated as the sizes given are incorrect.

I know how IBI is calculated and I think that it is based on CONSTRUCTED area rather than the size of the plot.  Can anyone tell me what I need to measure?  I have a mixture of internal space, covered terraces and open terraces.  On a document from the town hall I have the following:

Impuesto sobre Bienes Inmuebles (known as IBI) is fixed by the local authorities, depending on both the value of the land and of the property itself.

The Tax is based on the Cadastral value of your property This value is found on your IBI bill.

Each year the Cadastral value increaces by 2%. If like me you have owned your property for 8 years the Town Hall will value my property at 16% above the purchase price.

With regards to land area this will be shown on your Escritura which the Town Hall and land registary have access to and base there assesment on.

If you have completed any changes to your property the permission value will be added to the value of the property.

I am sure I will be corrected if I am wrong.

If not. I hope this helps.

Thanks for the reply but you didn't answer my question.  I already knew what you told me (technical stuff).

Your tax liability is calculated by reference to the cadastral value of your property. The cadastral value ("valor catastral") is equivalent to the UK rateable value.  The total cadastral value ("valor catastral total") for the year in question is the figure we need.  This is made up of two components: the land value ("valor catastral suelo") and the property value ("valor catastral construccion").  It does not relate directly to the purchase price or market value of your property.
 

I.E. What you should earn, theoretically , in renting it.

It varies from area to area.  Here , the value of land is not included but assessed seperately. historically the valor catastral is 20 x less than the market value but unfortunately this is now not the case as shown in areas of the CDS where the revaluations have left people with values of 3 or 4 x more than the market value !

My own value is 17 x less than purchase price & probably 60 x less now than market value, even in the 'crisis' market !

When they revalued here in 2005 ours went down by 30% as they reduced the valor catastral & the % payment amount. No idea why & it is correct as I went in numerous times thinking a mistake had been made !

In this area there is no automatic yearly increase  but is assessed by the local council.

Certainly in Lorca covered terraces and patios ,within the frame work of the build, are measured at 50% of area involved. 

However it appears there are no set rules in calculating  IBI in Spain so perhaps your architect would be the best person to ask.

 IBI is taken from Catastro data and not from escritura data

This is bit like the chicken and the egg.

What ever it is when my friend enquired about getting the error corrected she was told by the town hall

                                                                                                "  she cannot just submit the correct measurements to the town hall to get the error remedied. It must be done by a qualified surveyor accepted by the town hall. "