This morning, we are announcing our recommended offer for a combination with Warehouse REIT. We believe this is an attractive opportunity to enhance and deepen our overall logistics offer, building on the work that we've done to date in the urban logistics market and complementing our high-quality big box portfolio. We've outlined here some of the benefits as we see it, including cementing our leading position as the only listed REIT exclusively focused on U.K. logistics, increasing our exposure to urban logistics, which has attractive rental growth potential: enhancing our client offer through a broader range of property types, providing a pipeline of compelling asset management opportunities, which our team is well placed to capture, delivering immediate cost savings, which are accretive to adjusted EPS, and further supports our market-leading cost ratio, fortress balance sheet, and advantageous cost of capital.
In summary, we believe this combination helps to unlock the full potential of Warehouse REIT to drive returns for both sets of shareholders. This combination aligns with our strategy, creating further asset management opportunities and enhancing our overall client offer. Warehouse REIT complements our portfolio with assets in key strategic locations where supply is constrained and brings a broader client base into the portfolio. The Warehouse REIT portfolio is highly reversionary and has many near-term opportunities to capture growing rental income. For our existing and potential new clients, it helps us to provide a greater range of building sizes and locations. Finally, as you know, Tritax Big Box has excellent development expertise, which we can use to unlock and maximize the value of Warehouse REIT's development land. We have outlined here the key details of the transaction.
We believe the nature of our offer is compelling for both shareholders of Warehouse REIT and Tritax Big Box. For Warehouse REIT shareholders, it provides an attractive combination of cash and shares that delivers both certainty and the opportunity to participate in the exciting future of the combined business. Given where we are in the property cycle more generally, this ability to continue to participate, we believe, is particularly attractive. For Tritax Big Box shareholders, the transaction is earnings accretive and broadly NTA neutral. It provides additional exposure to urban logistics at an attractive entry point and further enhances our earnings growth. For both sets of shareholders and investors more generally, it also keeps these critical assets to U.K. supply chains in the public markets. I'll now hand over to Bjorn.
Thank you, Colin. Through this combination, we are further deepening our offer in U.K. logistics, complementing and enhancing our existing portfolio with greater exposure to urban logistics and the attractive investment attributes these assets bring. The urban logistics market is compelling and is complementary to our existing big box portfolio. These fundamentals include increasing value usage by occupiers of urban logistics assets, an acute shortage in supply due to a combination of the loss of logistics space in urban areas from higher value alternative uses, high land values and build costs, which combined can make new development unviable, versus the ongoing strong occupational demand leading to low vacancy rates, particularly in the key regional markets. These market dynamics combine to support healthy levels of rental growth, which are forecast to be more than 4.4% annually over the next four years.
This transaction really builds on our expertise and investment in urban logistics assets, which offer accelerated reversionary capture through the combination of open market rent reviews and short lease profile. On this slide, we set out the timeline of our urban logistics strategy and execution. Based on the market fundamentals outlined previously, we made the strategic decision to increase our exposure to urban logistics in early 2022. However, pricing at that time did not offer the right entry point, so we remained patient and disciplined in our approach. In 2023, following a repricing of logistics assets and some more motivated sellers, we selectively acquired two urban investments, the Junction Six estate in Birmingham and another in Enfield, North London. Building further on this, in 2024, we completed the acquisition of UK Commercial REIT, which provided an urban logistics portfolio of scale at a very attractive entry point.
We've made excellent progress integrating and maximizing value from these assets by increasing contracted rent by over 12% in the roughly 12 months of ownership. At Tritax Management, we've been investing significantly over the last three years in our asset management capabilities, enhancing our analytics and growing our team to support the delivery of this performance through direct and active engagement with customers. Warehouse REIT offers further opportunities for us with a compelling over 7% reversionary yield from current market rents, plus the potential of further income generation from the development land at Radway Green Crewe. There is planning approval in phase one for just over 800,000 sq ft, and phase two could accommodate over 1 million sq ft, subject to planning.
On this slide, we have outlined the relative size and composition of our portfolio GAV compared with Warehouse REIT, which would represent about 11% of the combined business, contributing around GBP 43 million per annum to the contracted rent and adding 6.9 million sq ft to our portfolio area. You will note that the potential reversionary capture is 25%, complementing the income growth potential. On this slide, we have outlined the positive impact of combining the portfolios of Warehouse REIT with Tritax Big Box, which further highlights its complementary nature. Turning to look in more detail at the Warehouse REIT assets, there is a breadth of estate sizes spread across all the major urban logistics regions in the U.K.
The portfolio can be described as more granular than the Tritax Big Box portfolio with 60 estates, which comprise just over 600 individual units, where some tenants have taken multiple units, resulting in 409 different occupiers. Its client composition is well diversified, both by number and uses, adding many new names to our portfolio, but with some common overlap where synergies can be gained. Occupancy levels across the portfolio sit at around 94%, which is typical with this number of units and allows for the market rent reversion across the various estates to be captured. Critically, these types of urban logistics assets and the supportive market fundamentals make them competitive to acquire in the open market, which this combination would facilitate. Now we provide more detail on the composition of the portfolio, with the concentration to key logistics hubs in the U.K. outlined in the chart on the left.
80% of the portfolio is multi-let and offers attractive short-length leases, of which 90% are open market rent reviews. This income profile allows for a lot of upside to come from the portfolio, which is 25% reversionary. Most importantly, we expect 43% reversion to be captured via the rent reviews in the next 12 months, as we have highlighted in the graph below. This is ignoring the further rent capture from the vacant space and potential development pipeline. This more granular and highly reversionary portfolio really does create a pipeline of exciting asset management opportunities to accelerate rental income capture over the near term. Looking closer now at the Warehouse REIT portfolio characteristics compared to the existing Tritax Big Box portfolio, we demonstrate how it enhances our logistics portfolio and provides accelerated opportunity to capture rental reversion.
As you can see, the key benefits in the Warehouse REIT further add to our portfolio exposure in the core regions of the U.K., adding further smaller units to our overall portfolio size bands. With Warehouse REIT primarily open market rent review structure, if we are successful on combination, the weighting of the open market reviews in the combined portfolio would be nearly 50%. Overlaying the Warehouse REIT reversion profile against our own existing one shows the extent of the opportunity we have to capture significant near-term rental reversion, which will drive attractive rental income growth for our shareholders. Thank you, and now over to Frankie.
Thank you, Bjorn. In addition to the strategic benefits of the acquisition, we believe it also has a number of compelling financial benefits over the short, medium, and longer term. Firstly, looking at the cost savings, the transaction is expected to generate significant immediate cost savings totaling GBP 5.5 million per annum, or 70% of the administrative costs of Warehouse REIT as last reported. This is split between GBP 4.9 million of management fee savings following the unification of the management contract under the existing Tritax Management IMA, along with GBP 0.6 million of other administrative cost savings. Given Big Box's investment-grade credit rating, the margin at which it borrows across its corporate RCF is currently 65 basis points lower than that of Warehouse REIT. Over time, we therefore expect this cost of capital benefit to feed through into efficiencies across the Warehouse REIT debt financing.
Our strong balance sheet has allowed us to put forward an offer which we believe has an attractive mix of shares and cash. As a result, and given the relatively small size of the Warehouse REIT indebtedness, this only modestly impacts on Big Box's balance sheet KPIs. Given our conservative leverage position, looking at the pro forma balance sheet post the acquisition, the LTV remains below 30%. When adjusted for the cash component of the transaction, we remain firmly within our medium-term LTV target of below 35%. Many of the key financing metrics of the two businesses are aligned. The pro forma combined average cost of debt at 3.2% and the average maturity at 4.4 years remains relatively unchanged when compared to Big Box on a standalone basis. The combined debt book will also remain significantly fixed or hedged at 93%.
We would also expect Big Box's BAA1 credit rating to apply to the larger group, given all of these combined financing metrics remain in line with the BAA1 credit, if not higher. Following our refinancing of certain nearer-term Big Box debt maturities, as announced on the 23rd of June, the combined group presents a well-staggered debt maturity profile, with its earliest maturity not falling due until December 2026. To summarize, the acquisition has a number of strategic and financial benefits. Firstly, enhanced earnings. The cost synergies are expected to lead to accretion to adjusted EPS in the first four-year post-completion, which will support our future dividend progression. Secondly, there is a significant opportunity to unlock value via asset management, including a significant rental reversion of 25% within the Warehouse REIT portfolio, with lease profiles that facilitate the shorter-term capture of this embedded income.
Thirdly, based on our assessment of the portfolio, we expect to deliver returns materially above Big Box's cost of capital over the medium term. By acquiring at the corporate level, we are also mitigating significant direct property transaction costs by acquiring in this way. Finally, it increases Big Box's exposure to a structurally supported market with attractive long-term fundamentals and prospects for future rental growth. As we will explain in more detail next week at our investor seminar, all of this supplements Big Box's existing growth drivers, which we believe has the potential to increase adjusted earnings by 50% by the end of financial year 2030. Thank you, and now back to Colin.
To conclude, we believe that this combination makes good strategic and financial sense for both Warehouse REIT and our shareholders.
Our offer delivers both the certainty of cash, but also the potential future upside that we believe is inherent in our business. At this point in the property cycle, our offer also provides the opportunity for public market investors to continue participating in U.K. logistics real estate. The combination is strategically attractive, increasing Tritax Big Box REIT's exposure to the urban logistics market. It increases our end-to-end U.K. logistics offer, allowing us to serve the full range of our clients' needs. It also helps unlock value from Warehouse REIT through leveraging Tritax's expertise and knowledge gained over many years investing in and managing U.K. logistics real estate. Finally, it supports growth, driving returns for shareholders through the capture of significant rental reversion and attractive development opportunities. In simple terms, we think that this combination helps to unlock the full potential of Warehouse REIT.