Querying SQL Server extended properties for tables/views and its columns
If you wish to return Column level Extended Properties for all columns in the database, use this query:

SELECT
    ObjectType = o.type_desc,
    SchemaName = SCHEMA_NAME(o.schema_id),  
    ObjectName = o.name, 
    ColumnName = clmns.name,
    ExtendedPropertyName = p.name,
    ExtendedPropertyValue = CAST(p.value AS sql_variant)

FROM sys.objects AS o
INNER JOIN sys.all_columns AS clmns
    ON clmns.object_id = o.object_id
INNER JOIN sys.extended_properties AS p
    ON p.major_id = o.object_id
    AND p.minor_id = clmns.column_id
    AND p.class = 1

WHERE o.type IN ('U','V') -- User Tables and Views


Query was taken (and modified) from Working with SQL Server Extended Properties.