The answer may depend on laws or regulations in your tax jurisdiction. But most places I am aware of, and general accounting advice, require item-by-item depreciation. Several reasons:
- Depreciation rates/schedules may differ by type or classification of asset
- Assets may be disposed of at different times, and it is advantageous to be able to remove a single item from the active list
- Tax law may change during the life of an asset, requiring different treatment at different points in the depreciable life of the asset (I once owned an office that went through four different schemes before it was fully depreciated)
- Specificity is always useful in accounting for clarity’s sake
Also be aware that not everything you purchase is a fixed asset. Typically, there is an expected life threshold, often one year, below which a purchase is considered merely a consumable supply, even though it might not normally be used up. A carpenter, for example, would be unlikely to list a hammer as a fixed asset, even though a favorite might remain in the toolbox for decades.