diff --git a/docs/adr/0003-where-to-develop-TUF-1-0-0.md b/docs/adr/0003-where-to-develop-TUF-1-0-0.md
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+# Develop TUF 1.0.0 in a subdirectory of the current TUF implementation
+
+* Status: accepted
+* Date: 2020-11-23
+
+Technical Story: https://github.com/theupdateframework/tuf/issues/1126
+
+## Context and Problem Statement
+
+The plan is to implement a refactored TUF (1.0.0) alongside the current
+code base, in order to not disrupt existing usage and keep providing
+a Python 2.7 client.
+
+We need to decide on the best place to do this development.
+
+## Decision Drivers
+
+* Developing the new code piecemeal
+* Continuing to make releases in the interim
+* Avoiding maintenance overhead
+
+## Considered Options
+
+Develop TUF 1.0.0:
+
+* In its own repository
+* In a separate development branch of the current TUF implementation
+* In the default branch, archiving the current implementation
+* In a subdirectory of the current TUF implementation
+
+## Decision Outcome
+
+Chosen option: "Develop TUF 1.0.0 in a subdirectory of the current TUF
+implementation", because we want to add the new TUF code gradually
+while keep maintaining the current implementation given limited
+maintenance resources.
+
+Once development of the new version is complete, we will transition
+from TUF 1.0.0 in a subdirectory to stand-alone TUF 1.0.0 by the following
+procedure:
+
+* flesh out tuf/api/*
+* implement tuf/client/new-updater.py
+* implement tuf/repository/*
+* \<iterate\>
+* git mv tuf/client/new-updater.py tuf/client/updater.py
+* git rm tuf/\*.py
+* tag 1.0.0
+
+## Pros and Cons of the Options
+
+Developing TUF 1.0.0 in a subdirectory of the current TUF
+implementation seems to have the least maintenance overhead compared to
+option 1 and 2, while allowing us to continue making releases with the
+old code unlike option 3.
+
+### Negative Consequences
+
+* In progress development in the default branch causes messiness
+  in plain sight.
+
+## Links
+
+* [Discussion of Python version support in TUF 1.0.0](https://github.com/theupdateframework/tuf/issues/1125)
+* [Discussion of deprecation policy for the pre-1.0, Python 2.7 supporting, code](https://github.com/theupdateframework/tuf/issues/1127)
diff --git a/docs/adr/index.md b/docs/adr/index.md
index 60cda8388c..424f13ea50 100644
--- a/docs/adr/index.md
+++ b/docs/adr/index.md
@@ -7,6 +7,7 @@ This log lists the architectural decisions for tuf.
 - [ADR-0000](0000-use-markdown-architectural-decision-records.md) - Use Markdown Architectural Decision Records
 - [ADR-0001](0001-python-version-3-6-plus.md) - Default to Python 3.6 or newer for new development
 - [ADR-0002](0002-pre-1-0-deprecation-strategy.md) - Deprecation strategy
+- [ADR-0003](0003-where-to-develop-TUF-1-0-0.md) - Develop TUF 1.0.0 in a subdirectory of the current TUF implementation
 
 <!-- adrlogstop -->