I was raised by hoarding parents who would be classified among the worst of any examples I've seen on hoarder TV programs. Some rooms were filled above the eye level, 6' high or more, with no clear pathways, the doors unable to be opened without pushing hard to let a thin person barely squeeze in. You could get trapped in a room and have to dig yourself out if something nearby fell into the little remaining space allowing the door to open at all.
A large portion of the clutter was due to materials and supplies intended to be used for my parent's occupations in Christian ministries to hospital patients and juvenile inmates. Much of these materials were regularly being donated by well-meaning supporters of the ministry who didn't know the havoc and chaos it was causing with my parents who never refused a donation. In fact, my parents would make regular trips to pickup the materials. The constant inflow was a serious problem because the distribution outflow was impossible for them to maintain, since the materials were mostly seasonal (Christmas, Easter, etc.).
I hired professional organizers to help solve the problem. Although it took a lot of time and expense, I was able to accomplish the decluttering using several solutions. We had purchased a large storage shed building several years before, which they filled up too. But we were able to use that shed as a key part of the solution. We decluttered and organized it, then dedicated that space exclusively for ministry materials, while at the same time declaring the house to be off limits for any such item. We also stopped all donations until the storage shed had the space allocated for it.
This created a clear separation between home and work items. Ministry items were not allowed in the home. Donations were never recontinued because they had more materials than they could ever use for the remaining years of their ministry. Their decline in mobility as they aged provided additional motivation to not restart donations. They had tried in the past to dedicate one bedroom for ministry items, but that wasn't successful because they filled it up without removing the items from the rest of the house.
The shed was able to free up one whole room to be organized and cleared as a critical first step to give working space and shelve for future organizing. Then we used that space to organize the rest of the house, one room at a time.
The process took about 2 years because I knew that it wouldn't be successful if we didn't truly organize the newly cleared space and reinforce organizing behaviors on a regular basis, quickly addressing any return to hoarding activities. I had the organizer visit every Saturday for 4-8 hours.
I'm very satisfied that I spent the time and money to do this because I was able to have my parents stay at home for home care and treatment as much as possible, with only brief temporary stays in rehab centers instead of putting them into long term convalescent or hospice facilities. Home care would've been impossible in their cluttered hoarding state.