As filed with the Securities and Exchange Commission on August 11, 2004

Registration No. 333-115436

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Amendment No. 6 to

Form S-11

FOR REGISTRATION UNDER THE SECURITIES ACT OF 1933

OF SECURITIES OF CERTAIN REAL ESTATE COMPANIES

EXTRA SPACE STORAGE INC.

(Exact Name of Registrant as Specified in its Governing Instruments)

2795 East Cottonwood Parkway, Suite 400

Salt Lake City, UT 84121

(801) 562-5556

(Address, Including Zip Code, and Telephone Number, including Area Code, of Registrant’s Principal Executive Offices)

Kenneth M. Woolley

Chairman and Chief Executive Officer

Extra Space Storage Inc.

2795 East Cottonwood Parkway, Suite 400

Salt Lake City, UT 84121

(801) 562-5556

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copies to:

Approximate date of commencement of proposed sale to the public:    As soon as practicable after this registration statement becomes effective.

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If delivery of this prospectus is expected to be made pursuant to Rule 434, check the following box.  ¨

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to Section 8(a), may determine.

The information in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and we are not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

20,200,000 Shares

Common Stock

This is our initial public offering of shares of our common stock. We are offering all shares of our common stock. All of the shares being offered by this prospectus are being sold by us. No public market currently exists for our common stock. We intend to elect to qualify as a real estate investment trust, or REIT, for U.S. federal income tax purposes.

The initial offering price of our common stock is expected to be between $13.00 and $15.00 per share. Our shares have been approved for listing subject to official notice of issuance on the New York Stock Exchange under the symbol “EXR.”

The shares of our common stock are subject to certain restrictions on ownership and transfer intended to preserve our qualification as a REIT. See “Description of Stock—Restrictions on Transfer.”

Investing in our common stock involves a high degree of risk. Before buying any shares, you should read the discussion of some risks of investing in our common stock in “ Risk Factors” beginning on page 21, including, among others:

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The underwriters may also purchase up to 3,030,000 additional shares of common stock from us at the public offering price, less underwriting discounts and commissions, within 30 days from the date of this prospectus. The underwriters may exercise this option only to cover over-allotments, if any.

The underwriters are offering the common stock as set forth under “Underwriting.” Delivery of the shares of common stock will be made on or about                     , 2004.

A.G. Edwards

Banc of America Securities LLC

Raymond James

RBC Capital Markets

Wells Fargo Securities, LLC

[PICTURES, TEXT AND GRAPHICS FOR INSIDE FRONT COVER]

You should rely on the information contained in this prospectus. We have not authorized anyone to provide you with information different from that contained in this prospectus. We are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where offers and sales are permitted. The information contained in this prospectus is accurate only as of the date of this prospectus, regardless of the time of delivery of this prospectus or any sale of common stock.

TABLE OF CONTENTS

Through and including                      , 2004 (the 25th day after the date of this prospectus), federal securities laws may require all dealers that effect transactions in our common stock, whether or not participating in the offering, to deliver a prospectus. This is in addition to the dealers’ obligation to deliver a prospectus when acting as underwriters and with respect to their unsold allotments or subscriptions.

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Prospectus summary

You should read the following summary together with the more detailed information regarding our company, including under the caption “Risk Factors,” and the historical and pro forma financial statements, including the related notes, appearing elsewhere in this prospectus. Unless the context otherwise requires or indicates, references in this prospectus to “Extra Space Storage,” “we,” “our company,” “our” and “us” refer to Extra Space Storage Inc., a Maryland corporation, together with our consolidated subsidiaries, including Extra Space Storage LP, a Delaware limited partnership, which we refer to in this prospectus as our “operating partnership,” Extra Space Management, Inc., a Utah corporation, which we refer to in this prospectus as our “taxable REIT subsidiary,” and Extra Space Storage LLC, a Delaware limited liability company, and its affiliates which we refer to in this prospectus as the “Extra Space Predecessor” or “our predecessor.” Unless the context otherwise indicates, the information about our company assumes that the formation transactions described in this prospectus have been completed. In addition, the information contained in this prospectus assumes that the underwriters’ over-allotment option is not exercised, and the common stock to be sold in the offering is sold at $14.00 per share, which is the mid-point of the price range indicated on the cover page of this prospectus. References to “common stock” exclude contingent conversion shares, or CCSs, unless otherwise indicated.

OVERVIEW

We are a fully integrated, self-administered and self-managed real estate investment trust formed to continue the business commenced in 1977 by our predecessor companies to own, operate, acquire, develop and redevelop professionally managed self-storage properties. Since 1996, our fully integrated development and acquisition teams have completed the development or acquisition of more than 100 self-storage properties and continue to evaluate a range of new growth initiatives and opportunities for our company. To enable us to maximize revenue generating opportunities for our properties, we employ a state-of-the-art proprietary web-based tracking and yield management technology called STORE. Developed by our management team, STORE enables us to analyze, set and adjust rental rates in real time across our portfolio in order to respond to changing market conditions.

Upon completion of the offering and the formation transactions, we will own and operate 136 self-storage properties located in 20 states, 118 of which are wholly owned and 18 of which are held in joint ventures with third parties, and we also manage for unaffiliated third parties an additional nine properties. Our properties are generally situated in convenient, highly-visible locations regionally clustered around high-density, high-income population centers, such as Boston, Chicago, Los Angeles, Miami, New York/Northern New Jersey and San Francisco. Our properties contain an aggregate of approximately 8.9 million net rentable square feet of space configured in approximately 84,800 separate storage units. As of May 31, 2004, our stabilized portfolio (which consists of 108 properties) was on average 87.4% occupied, while our lease-up portfolio (which consists of 28 properties) was on average 62.4% occupied. We consider a property to be in the lease-up stage after it has been issued a certificate of occupancy but before it has achieved stabilization. We consider a property to be stabilized once it either has achieved an 85% occupancy rate, or has been open for four years. Over the next 24 months, we expect our lease-up properties to achieve 85% occupancy, which we believe is in-line with lease-up periods typical in the self-storage industry.

As of May 31, 2004, we had more than 70,000 tenants leasing storage units at our 136 properties, primarily on a month-to-month basis, providing us with flexibility to increase rental rates over time as market conditions permit. Although our leases are short-term in duration, our typical tenant tends to

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remain at our properties for an extended period of time. For properties that were stabilized as of May 31, 2004, the average length of stay for our tenants was approximately 16 months.

Members of our senior management team have significant experience in all aspects of the self-storage industry, with an average of more than nine years of industry experience. Our senior management team has collectively acquired and/or developed more than 176 properties during the past 25 years for our predecessor and other entities. Kenneth M. Woolley, our Chairman and Chief Executive Officer, and Richard S. Tanner, our Senior Vice President, East Coast Development, have worked in the self-storage industry since 1977 and led two of the earlier self-storage facility development projects in the United States. In addition, eight members of our management team have worked together for our predecessors for more than five years. Members of this management team have guided our predecessor through substantial growth, developing and acquiring $699.0 million in assets since 1996. Our senior management team funded this growth with internal funds and more than $245.0 million raised in private equity capital since 1998, largely from sophisticated, high net-worth individuals and institutional investors such as affiliates of Prudential Financial, Inc. and Fidelity Investments. Mr. Woolley, along with Spencer F. Kirk, one of our directors, and our senior executive officers may be considered promoters with respect to the company. See “Management—Directors and Executive Officers.”

Our principal corporate offices are located at 2795 East Cottonwood Parkway, Suite 400, Salt Lake City, Utah 84121, our website address is www.extraspace.com and our telephone number is (801) 562-5556. The information included in our website is not considered to be a part of this prospectus. Upon completion of the offering and the formation transactions, substantially all of our business will be conducted through Extra Space Storage LP, our operating partnership, and our primary assets will be our general partner and limited partner interests in Extra Space Storage LP. This structure is commonly referred to as an umbrella partnership REIT, or UPREIT.

THE SELF-STORAGE INDUSTRY

Self-storage refers to properties that offer do-it-yourself, month-to-month storage space rental for personal or business use. Self-storage provides a convenient way for individuals and businesses to store their possessions, whether due to a life-change, or simply because of a need for extra storage space. According to the 2004 Self-Storage Almanac, there were approximately 37,000 self-storage properties in the United States in 2003 with an average occupancy rate of 84.6%, compared with approximately 19,500 U.S. self-storage properties in 1992 with an average occupancy rate of 84.8%. As population densities have increased in the United States, there has been an increase in self-storage awareness and development, which we expect will continue in the future.

The self-storage industry is also characterized by fragmented ownership. According to the 2004 Self-Storage Almanac, as of December 31, 2003, the top five and top 50 self-storage companies in the United States owned only approximately 10.2% and 15.7%, respectively, of the total U.S. self-storage properties. We believe this market fragmentation will provide opportunities for continued consolidation in the self-storage industry, particularly for well-capitalized, publicly-traded companies with experienced acquisition teams.

We have found that the factors most important to tenants when choosing a self-storage site are a convenient location, a clean environment, friendly service and a professional helpful staff. Our experience also indicates that successfully competing in the self-storage industry requires an experienced and dedicated management team that is supported by an efficient and flexible operating platform that is responsive to tenants’ needs and expectations.

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COMPETITIVE STRENGTHS

We believe we distinguish ourselves from other owners, operators and developers of self-storage properties in a number of ways and enjoy significant competitive strengths, which include:

Our properties are generally situated in convenient, highly-visible locations clustered around large population centers such as Boston, Chicago, Los Angeles, Miami, New York/Northern New Jersey and San Francisco. The clustering of our assets around these population centers enables us to reduce our operating costs through economies of scale. At the same time, we believe that the significant size and overall geographic diversification of our portfolio reduces risks associated with economic downturns or natural disasters in any one market in which we operate.

We have developed and utilize a comprehensive centralized approach to property and operational management to maximize the operating performance of our properties. We use STORE to support all aspects of our property management operations, enabling our management team to centrally analyze, set and adjust rental rates in real time on a case-by-case basis across our entire portfolio to maximize revenue-generating opportunities.

Our property management and operations groups are supported by our marketing team that provides sales, marketing and advertising support for our properties and operations. We employ highly targeted direct response marketing programs, such as direct mail and coupon mailers, in combination with more broad-based marketing initiatives such as advertising in the Yellow Pages and on the internet.

Our fully-integrated development and acquisition teams have completed the development or acquisition of more than 100 different self-storage properties since 1996. In addition, we have entered into agreements to acquire 29 properties from unaffiliated third parties upon completion of the offering. We believe that we have developed a reputation as a trusted and reliable buyer. In addition, following completion of the offering and the formation transactions, we expect to be one of only two publicly-traded REITs in the self-storage industry that is organized in the UPREIT format, which will enable us to acquire new properties from tax-deferred transactions.

Our Chairman and Chief Executive Officer, Kenneth M. Woolley, and our co-founder, Richard S. Tanner, have been in the self-storage business for more than 25 years. Together, they have acquired or developed more than 176 self-storage properties. Our senior management team has an average of more than nine years of self-storage experience.

We have developed and/or acquired more than 70 properties since 1999 employing strategic joint ventures with nationally-recognized institutional investors such as affiliates of Prudential Financial, Inc. and Fidelity Investments. We believe our reputation for quality within our industry, and our management and development expertise, make us an attractive strategic partner for institutional investors.

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BUSINESS AND GROWTH STRATEGIES

Our primary business objectives are to maximize cash flow available for distribution to our stockholders and to achieve sustainable long-term growth in cash flow per share in order to maximize long-term stockholder value. Our business strategy to achieve these objectives consists of the following elements:

We will seek to maximize revenue generating opportunities by responding to changing local market conditions through interactive yield management of the rental rates at our properties.

We intend to selectively acquire, for cash or by utilizing units in our operating partnership as acquisition currency, privately-held self-storage portfolios and single self-storage assets in our target markets.

We plan to continue to expand also by selecting and developing new self-storage properties with cost-effective, appealing construction in desirable areas based on specific data, including visibility and convenience of location, market occupancy and rental rates, market saturation, traffic count, household density, median household income, barriers to entry and future demographic and migration trends. As of July 15, 2004 we had 12 undeveloped parcels of land under contract that we believe are suitable for new property developments and are proceeding with the requisite due diligence for these properties. We also have a right of first refusal with respect to sales of the interests in the 13 early-stage development properties owned by Extra Space Development LLC and the two early-stage lease-up stage properties owned by third party individuals as well as certain of our executive officers and directors. We also are currently reviewing more than 22 other sites that we believe may also be suitable development candidates.

We plan to grow our business by continuing our development activities in conjunction with our joint venture partners, while mitigating the risks normally associated with early-stage development and lease-up activities. Where appropriate, we will also seek to acquire properties in a capital efficient manner in conjunction with our joint venture partners.

SUMMARY RISK FACTORS

You should carefully consider the matters discussed in the section “Risk Factors” beginning on page 21 prior to deciding whether to invest in our common stock. Some of the risks include:

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OWNED PROPERTIES

Upon completion of the offering and the formation transactions, we will own and operate 136 self-storage properties located in 20 states, 118 of which are wholly owned and 18 of which are held in joint ventures with third parties. The following tables set forth summary information regarding our 108 stabilized and our 28 lease-up properties:

Stabilized Property Data

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Lease-Up Property Data