Housebuilders have a habit of treating their homebuyer's Subject Access Requests with contempt.
Many are returned incomplete and/or redacted to such an extent as to be meaningless.
I am pleased you complained to the ICO and from my own experience, their response is somewhat tardy to say the least! I think the phrase "Data that relates to you as an individual" would, for most reasonable people, also include everything in their homebuyer "plot file" excluding anything that either discloses the data of others or that which is commercially sensitive.
All data supplied should also contain details of e mails (even if internally - in-house) which name you and all written logs of any telephone calls.
The main problem here in my opinion, is the ICO don't have sufficient staff, of the right calibre, to follow through on the actual rules and serve only to bat away most complaints from individuals concerning data protection.
On a personal level, I have, and still am, trying to get Currys/PCWorld to delete my personal details from their files and systems and in three years since my original complaint, the ICO have been of very little help if any.
On a more positive note the New Homes Ombudsman should be operational at some point this year however the Coronavirus Pandemic may be a convenient excuse for government and the industry to delay it yet further.
Not all housebuilders are as petty and pedantic as you can see from the photo.
You really should name your housebuilder.
I question exactly what they are trying to keep from you finding out!