OSS Add-On Question and Software Suggestions?

[Coast2Coast]

Hello,
I'm looking for a software that is easy to learn to help design pressurized on-site septic systems. I am a consulting firm of one, so cheaper is better (everything seems to have gone to subscription), but user friendly is what I'm most looking for. Most of my projects are rural = barn bathrooms, ADU's or expansions to existing OSS.

For anyone who designs OSS, opinions on the pros and cons of adding a second or to replace/add on to an existing system? We are upgrading a 3 bedroom house to include an old workshop turned a 1 bedroom (but min 2 bdrm design) granny flat. Client reasonably feels that removing the existing is unnecessary and wants to add-on to existing. I'm also looking for a detail or resources that show options for how a connection to add-on could work as opposed to a separate system or removing existing 900 gallon tank (for a 1500).

Thank you for any help/guidance!
Cheers!

 

[AHickman]

This post is a little old, but I'll answer it anyways. I design large commercial septic systems for a living, so I don't think I can help much for the first paragraph (autocad and excel though).

As for the second part, never remove tanks, crack and fill them. Is this a conventional system, or aerobic? Gravity/pumped?