No statute requires you to hire anyone. The IRS accepts that taxpayers can allocate their own basis, and the Cost Segregation Audit Techniques Guide even catalogs the approaches it encounters, from detailed engineering analyses down to rule-of-thumb percentages. But the same guide is explicit that reliability tracks methodology, and self-prepared percentage allocations sit at the bottom of the hierarchy. If you are examined, the burden of substantiating every allocation is yours.
What you can reasonably do yourself
Some segregation never needed a study. If your closing statement or invoices separately state costs, say appliances, furniture for a furnished rental, or a fence installed after purchase, you can and should book those directly as 5-, 7-, or 15-year assets at their documented costs. Renovations you paid for item by item are similar: a $14,000 invoice for new kitchen appliances and a $9,000 landscaping contract are their own assets with their own class lives under IRS Pub 946, no engineer required.
Where DIY breaks down
The value in cost segregation is mostly in costs that were never separately stated: the dedicated electrical runs, the plumbing serving specific equipment, the share of a lump-sum purchase price that is really carpet, cabinetry, and site work. Splitting those out requires component takeoffs and cost estimation, which is engineering work. Pulling a 30% figure from an article and applying it to your basis is exactly the rule-of-thumb method the Audit Techniques Guide flags, and an overstated allocation that produces an underpayment can trigger the 20% accuracy-related penalty under IRC section 6662 on top of back taxes and interest.
A cautionary example
An owner self-allocates 35% of a $450,000 suburban rental using a percentage from a forum post. The property is a simple structure where a defensible study would have found closer to 20%. The difference, about $54,000 of basis taken as bonus depreciation under IRC section 168(k), is pure exam exposure with no documentation behind it. If the IRS examined and reduced the allocation, the tax, interest, and potential penalty would likely exceed what a professional study would have cost in the first place.
The sensible sequence
Run a screen first. Amadae's cost segregation screen models the likely reclassification and first-year benefit before you spend anything, which tells you whether an engineering study pencils at all. If it does, use an engineering-based provider for the study itself; Amadae coordinates the study, books the results into your depreciation schedule, prepares the Form 3115 catch-up when the property was placed in service in a prior year, and files the return that uses it. DIY the parts with paper trails. Leave the lump-sum carve-up to engineers.