If an asset is owned by you personally, it cannot be contributed into the Solo 401k or re-titled into the Solo 401k trust.
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The only way to put assets into the Solo 401k is through rollovers from another retirement plan. Contributions only come from earned income of the business adopting the Solo 401k.

For example – if you own an office building, you cannot simply contribute the asset into the Solo 401k (transfer of title). Even if the building was purchased from income you got from running your business, it was not originally a tax-deferred investment and so it cannot be contributed.