Pretty certain it's part of the original build - things like the cross joists matching the main joists, both in timber and workmanship style.
And if you look closely, there's a couple of boards marked that have the bottom part of the groove removed so they will lift out. I can still get under when I need to I'm not putting all this effort in and leaving things so it needs butchering next time I need access.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local