Breach of Developmental Contract and poor Structural construction

I have bought a property from Galencia at Cedar Acres Estate Craigavon which is a sectional title. The property now shows signs of poor structural defects with regards to internal plumbing and structure. Pipes coming from my bathroom shower,kitchen ,washing machine and bathroom basin are all 25mm pipes and these four pipes are all feeding into one 25 mm pipe leading outside.

This in turn means that there is a backflush of water between the shower, kitchen , washing machine and bathroom basing back into the house. I spent money on plumbers thinking this was just a small blockage only to get this feedback. I now went back to Galencia Maintenance(Herman) who came to my house and saw what was happening, he committed to bringing someone and making a permanent fix. He has not come back and is ignoring my calls and I am now getting the feedback that there is nothing they can do from their side it is my problem now. I further called Galencia after sales and spoke to a certain Mr Tom Young who also in a nutshell told me that there is nothing they can do.

I need help because structure plumbing and electrical structure are plans that are submitted for approval and Galencia is obviously breaching these and making this the clients issue to deal with.

I am paying mortgage on this house that is extremely unpleasant to stay in and even had to move out until I get the issue sorted. Galencia is benefiting from my money and they are taking advantage of South Africans and giving them terrible constructions.