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A series limited liability company, commonly known as a series LLC and sometimes abbreviated as SLLC, is a form of a limited liability company that provides liability protection across multiple "series" each of which is theoretically protected from liabilities arising from the other series. In overall structure, the series LLC has been described as a master LLC that has separate divisions, which is similar to an S corporation with Q-subs.
The concept of the series LLCs was special interest legislation first introduced to help the mutual fund industry avoid filing multiple SEC filings for different classes of funds. Instead the concept was to use one entity for all funds so that the SEC filing would be under one umbrella, but activities of the individual funds could be conducted with a large degree of autonomy. The concept is similar to the segregated portfolio companies and protected cell companies which have existed for quite some time. The segregated portfolio companies exist in countries such as Guernsey, British Virgin Islands, Bermuda, the Cayman Islands, Mauritius and Belize.
This method of liability segregation was first called the "Delaware Series LLC" because the first state to enact this legislation was Delaware in 1996. As of April 2005, Iowa  and Oklahoma already had passed similar acts. Later in 2005, Illinois and Nevada followed suit. Tennessee and Utah  passed legislation effective in 2006. Wisconsin passed a stripped-down version of the series LLC legislation.
The utility of a Series LLC may be explained by a comparison to the alternative. Many form an LLC in order to protect personal assets from a legal claim relating to their real estate investment or business liabilities. Retrieved 2009.01.16.</ref> Additional liability protection may be gained by properly forming and maintaining a separate LLC to hold each property or business entity. By forming a separate LLC to own and hold each legally titled separate property or business entity, theoretically only the assets owned by a specific LLC would be subject to claims or lawsuits arising against that LLC. However there are costs and administrative burdens associated with properly forming, qualifying and maintaining each separate LLC. Another option may be to form multiple series or "cells" if permitted under applicable laws. Although each cell of a Series LLC can own distinct assets, incur separate liabilities, and have different managers and members, a Series LLC may be able to pay a single set of annual state fees and may be able to file one income tax return each year. In addition to the administrative streamlining, the key value is that liability incurred by one unit does not cross over and jeopardize assets titled in or allocated to other subsidiary units of the same Series LLC.
In several jurisdictions, the procedure for adding and deleting series is uncomplicated. Additional series can be formed or dissolved without any public filing by simply amending the Series' “limited liability company agreement” (equivalent to an operating agreement for other LLCs). Under Delaware law, any particular series may be dissolved by 2/3 approval of the ownership interests, or a simple majority if provided for in the operating agreement. Some jurisdictions, notably Illinois, do have a mechanism for public publication of series. Additionally Illinois states that each series is a separate entity, whereas Delaware is silent on whether each series is a separate entity. Most states with the series LLC have followed the Delaware model, rather than the model in Illinois which requires each series to be designated with the Secretary of State.
Until recently, Delaware did not clearly state that each series could sue, enter into contracts, etc. on its own, without the entire company being named in the lawsuit. Delaware clarified its legislation that a series can now enter into contracts, hold title to assets, grant liens and security interests and sue or be sued. In several other respects, series are not treated by Delaware as separate entities. For example, series are not separately registered and they cannot merge or consolidate with other entities, convert into other entity types or domesticate to another jurisdiction. The Delaware Division of Corporations will not provide a separate certificate of good standing for each series, but it will provide a certificate of good standing saying that the entire company is a series LLC (and not just a traditional LLC).
Illinois has restricted the rights given to the members of a series LLC to create new series because Illinois requires public filing. This has removed some of the cost savings of a series LLC. Illinois law specifically states that a series of an LLC “shall be treated as a separate entity to the extent set forth in the articles of organization,” and then also provides that each series may “in its own name, contract, hold title to assets, grant security interests, sue and be sued and otherwise conduct business and exercise the powers of a limited liability company…” The other five states that have enacted series legislation do not treat series as separate entities and do not allow series to enter into contracts or sue or be sued. Delaware further provides that to achieve the liability segregation that the series afford (the “internal shield”), the LLC must keep a separate set of records for each series, and to have a series enabling statement in its Certificate of Formation.
Acceptance by other jurisdictions 
The series LLC is becoming more widely used as a liability segregation technique as its tax treatment becomes clearer and its use spreads. To date, the inter-jurisdictional efficacy of portfolio segregation has not been widely tested and the lack of precedent in federal bankruptcy court in particular is a significant source of uncertainty. At the same time, tax treatment is becoming clearer. On January 18, 2008, the Internal Revenue Service issued Private Letter Ruling 200803004, which ruled that the Federal tax classification (i.e., disregarded entity or partnership or taxable association) is determined for each series independently. So, for example, if there is only one owner of series A, then series A can be a disregarded entity (assuming it does not elect to be taxed as an association). And if series B has two owners, then it will be treated as a partnership. The proposed Treasury Regulations § 301.7701-1(a)(5) published in September 2010 should become effective in 2012. The regulations are expected to provide that each series will be treated, for tax purposes, as a separate entity regardless of whether the series is considered a legally distinct entity under local law. This clarity has been welcomed by the legal and tax community. California has taken the position that it will only tax income from those series conducting business in California but that each such series will owe the annual franchise fee. Although the structure of LLCs vary in important ways, commentators have advanced opinions on how to minimize the chances of one series being held liable for liabilities of the entity as a whole or of another series. But they are just opinions and have not been held up in court:
- A separate bank account should be maintained for each series.
- All contracts, deeds, notes, etc. should be signed in the name of the series. Again, use something like "Abracadabra LLC, Blackacre Series only".
- Any loans between series should be properly documented.
- Any transactions between series should be conducted in an arms'-length manner at fair market prices using appraisals.
- Have each series file a fictitious business name statement in each county where it owns property. Each series should have its own name and the filing should emphasize the ownership of that series, for example, "Abracadabra LLC, Blackacre Series only". This is to put creditors on notice.
- Keep the assets and operations of each series separate from the other series. Each asset should be owned solely by one series. In other words, two or more series should not be co-owners of the same property.
- Make sure each series is adequately capitalized.
U.S. States and territories where a Series LLC can be formed
- Delaware (Limited Liability Company Act)
- District of Columbia
- Nevada (Nevada Revised Statutes)
- North Dakota
- Puerto Rico
- See, e.g., Thomas E Rutledge, Again, for the Want of a Theory: The Challenge of the "Series" to Business Organization Law, 16 Am. Bus. Law J. 311 at 313-15 (Summer 2009).
- Stein, Jacob (June 2006). "Advanced Asset Protection and Tax Planning with LLCs". Los Angeles Lawyer. Retrieved March 08, 2012.
- Iowa Code §490A.305
- Illinois General Assembly, Public Act 094-0607
- 2005 Statutes of Nevada, Page 2189
- Tenn. Code Ann. §48-249-309.
- Utah Code §48-2c-606
- Wis. Stat. §183.0504
- Illinois Secretary of State Certificate of Designation Instructions
- Stein, Jacob. "Creative Asset Protection Planning with LLCs". Retrieved March 08, 2012.
- Stein, Jacob (2011). A Lawyer's Guide to Asset Protection Planning in California. p. 232. ISBN 098397800X.
- Liquerman, Melissa C. (January 18, 2008). "Private Letter Ruling 200803004" (PDF). Internal Revenue Service.
- Proposed Regulation 75 FR 55699 (September 14, 2010)
- FTB Filing Guidelines for Series LLCs
- 6 Del. Code Ann. §18-215
- DC Code Ann. §29-802.06
- 805 ILCS 180/37-40
- Iowa Code Ann. §490A.305
- Substitute H.R. 2207 (2012)
- Minn. Stat. Ann. §322B.03, Subd. 6, 38, 44. Minnesota allows series LLCs, but does not specifically provide for a liability shield between the different series.
- Nev. Rev. Stat. §86.296
- N.D. Cent. Code §§10-32-17.5, 10-32-48, 10-32-56.5.a,10-32-56.7. North Dakota allows series LLCs, but does not specifically provide for a liability shield between the different series.
- 18 Okla. St. Ann. §§2005(B), 2054.4
- T.C.A. §48-249-309
- Texas Business Organizations Code §§101.601 to 101.621, 21.152(A),(C),(D), 21.153(A), 21.361(A)(2)
- Wis. Stat. Ann. §183.0504
- Wis. Stat. Ann. §183.0504. Wisconsin allows series LLCs, but does not specifically provide for a liability shield between the different series.
- Puerto Rico Laws Ann. Title 14, §3426(p)