@Karen Chenaille There are definitely people out there who will try the DIY approach with cost seg. The thing is you need to cross your fingers and hope you don't get audited. If the IRS actually looks at a DIY cost seg they'll immediately throw it out without any further review. Thing is reviewing depreciation is about at the bottom of the IRS priorities, so improper cost seg generally flies under the radar.
The IRS Chief Counsel released a memo stating that to perform cost segregation properly one must have knowledge of construction, construction techniques, design, auditing and estimation procedures. That means generally the only people truly qualified to do cost segregation are construction engineers or architects who have been trained in cost segregation and the pertinent tax code. That's why the first element in a quality study is preparation by an individual with expertise and experience. So a DIY method is not recommended unless you're actually qualified.
On the surface cost seg can seem simple enough to DIY. Say you remodeled a rental condo and have all the invoices for the rehab. It makes sense that you'd just need to figure out what qualifies for 5, 7 or 15 year depreciation from the rehab, keep the rest of the property on a 27.5 year schedule, setup a depreciation schedule and call it a day right? The problem with that is two-fold: first there are likely existing assets that are not replaced that qualify for shorter asset lives so you're leaving money on the table, and second you have no cost basis for the removed assets replaced in the remodel as they're intertwined with the initial building cost basis so you'd technically still be depreciating retired assets which is tax fraud.
It's also quite a bit more complicated than "this qualifies for 5 year and this doesn't qualify" since there isn't really a "list of every possible asset and it's depreciation life" most assets that qualify for 5 year fall under asset class 57.0 assets used in distributive trades and services. There are 6 tests called the WhiteCo factors (named after a tax court case on the subject) designed to figure out the permanence of individual assets to see if they qualify for short life treatment. You can even have the exact same asset and in one case it qualifies for 5 year and in another case it doesn't. A simple example is baseboards: if they're considered permanently attached to the wall like being glued on they're treated as a long life asset, it would damage the wall to remove it so it's like part of the wall; on the other hand if it's easily removable like being tacked or nailed onto the wall it qualifies for 5 year. That's just one out of a thousand different nuances that makes cost seg so complex.
Even among professional cost segregation firms there are huge differences in methodology and accuracy. The IRS describes 6 different methods with varying levels of accuracy. Many cost seg firms use a statistical sampling or computer generated modeling approach which is a bit cheaper since it's less work, but it's inherently inaccurate so it generally leaves money on the table or worse overstates qualifying assets. If that's what you're looking for it may be better to save the money and do it yourself since your guess may be as good as theirs. Accuracy is important so the detailed engineering from actual cost records and detailed engineering using cost estimation methods are the preferred approaches.
So you can DIY and realistically chances are quite low that you'd have any issues, but if the IRS reviews it the fines and penalties will significantly outweigh saving some money by not having it done professionally plus you're either gonna leave a lot on the table or overstate which is a surefire way to get audited. Having it done right gives you peace of mind on the audit front, you're not leaving money on the table, and the benefits will significantly outweigh the costs usually at least 3 to 1 but oftentimes it's 5 to 1 or 10 to 1 depending on the property.