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What is the Best Rate Guarantee (BRG ?)
If a lower rate at a Starwood Hotel is made available on a non-Starwood website or non-Starwood mobile application, upon its receipt of a claim that satisfies these Best Rate Guarantee terms and conditions, Starwood will honor that Competing Rate and provide the individual that submitted the valid claim one of the following rewards: (1) an additional 20% discount off the Competing Rate per room per night (up to a maximum of three rooms); or (2) 2,000 Starwood Preferred Guest® Starpoints® per room per stay (up to a maximum of three rooms).

BRG Wiki on Flyertalk

• You cannot BRG against a rate on a Starwood Website/App (hot escapes, promos, Gov. Rate etc.). It has to be a 3rd party website/App.
• Cancellation policy has no bearing on a BRG claim with one exception if the rate difference is within 1%.
"Starwood may deny claims where the difference between the Competing Rate and the rate on the Starwood Website is less than one percent; provided that Starwood will not use the fact that the difference between the Competing Rate and the rate on the Starwood Website is less than one percent to deny such claims if the cancellation policy for the Competing Rate is more favorable to the guest than the cancellation policy for the rate on the Starwood Website."
• A valid Starwood website reservation is required to submit a BRG (only exception is when there is no availability on the Starwood website)
• Only one claim may be submitted per reservation number (If your first claim is denied, you may submit a new claim for that reservation if you find another lower rate after you receive the denial email)
• Make sure to book a flexible rate on SPG.com. This will allow you to then cancel this booking if you find a lower Starwood rate later (like the hot escapes etc.) or if your travel plans change.
• You can BRG with a flexible rate against a prepaid rate. In other words, if expedia is offering a room for $100 prepaid, and spg.com shows a prepaid room for $150 or a flexible room for $200, you can book the FLEXIBLE spg.com rate and BRG it against the PREPAID $100 expedia rate. And you will receive the flexible cancel terms if the BRG is approved.
• If your comparable rate includes extras like breakfast or parking, make sure there is a room available on spg.com that includes the exact amenities. If there is the same room type with the same amenities on SPG.com, they will compare and if lower on third party, the claim may be approved for the room type and with the amenities. If there is the same room type available but no/different amenities offered, the claim may still be approved, but the rate will be room only.
• Room types must be the same for a BRG.
• If you have an approved and finalized claim and you find an even cheaper rate later, then you may submit a new BRG claim that is not connected to any existing reservation. If this BRG claim is approved, you will need to make a new flexible reservation on SPG.com. Once the process is completed, you will then email the BRG desk to cancel your first reservation, if it is within the cancellation deadline.
• If you find availability for the room type you want on a third party website/app but there is no availability for that room type or no availability for any room type on the Starwood Website, then this may be an approved claim if it meets all other terms and conditions. You will need to make sure that you specify the room type you actually want on the Starwood Website section and enter 0 in the rate and currency. This will ensure that the processing associates know that the room type you want is not available on the Starwood Website. You may also add additional comments near the end of the claim form indicating that there is no availability on the Starwood Website to help clarify.

Differing currencies
• "The Best Rate Guarantee department will approve a claim if there is a difference of 2.5% or greater where the currencies are different or a difference of 1% or greater if the currencies are the same."

Link to T&C
https://www.starwoodhotels.com/bestrate/terms.html

Practicalities During/After Stay
• Reconfirm your rate at check-in; normally the hotel will have the correct BRG rate in your reservation, but sometimes you will need to remind them.
• If you choose points, they will be credited after the stay; check in one week to see if they have posted. If not contact the BRG office to arrange to have them posted.

How to add a SNA to a BRG reservation
• Log into your SPG account
• Click on your stays to view our your current reservation
• Look for your BRG reservation and click on "send confirmation" link for the BRG reservation
• reload the page and you should be able to now select "apply SNA to this stay" link