Accumulate values into an array with Postgres Currently I have this query:
select 
    sum(case when my_table.property_type = 'FLAT' then my_table.price else 0 end) as Flat_Current_Asking_Price,
    sum(case when my_table.property_type_mapped = 'SEMIDETACHED' then my_table.price else 0 end) as Semidetached_Current_Asking_Price
from 
    my_table;

So if my_table has the values:
property_type | price
--------------+-------
FLAT          | 5000
SEMIDETACHED  | 9000
FLAT          | 6000

the query will return:
Flat_Current_Asking_Price | Semidetached_Current_Asking_Price
--------------------------+-----------------------------------
11000                     | 9000

How can I replace the sum to accumulate the values into arrays to get?
Flat_Current_Asking_Price | Semidetached_Current_Asking_Price
--------------------------+-----------------------------------
{5000, 6000}              | {9000}
If your PostggreSQL version is 9.4 or later use FILTER clause:

select
    array_agg(my_table.price) filter(where my_table.property_type = 'FLAT' ) as Flat_Current_Asking_Price,
    array_agg(my_table.price) filter(where my_table.property_type = 'SEMIDETACHED') as Semidetached_Current_Asking_Price
from 
    my_table;