diff --git a/PyTeX/build/build/builder.py b/PyTeX/build/build/builder.py
index 70b1993..24c3798 100644
--- a/PyTeX/build/build/builder.py
+++ b/PyTeX/build/build/builder.py
@@ -1,13 +1,14 @@
 from pathlib import Path
-from typing import Optional, Union, List
+from typing import Optional, Union, List, Tuple
 
 from .enums import PyTeXRootDirType
 from .pytex_config import PyTeXConfig
 from ...logger import logger
 from .constants import *
-from ..versioning.version_info.version_info import VersionInfo
+from ..versioning.version_info.version_info import VersionInfo, FileVersionInfo
 from .pytex_file import PyTeXSourceFile
 from .relative_path import RelativePath
+from .hashing import md5
 
 
 class PyTeXBuilder:
@@ -16,7 +17,6 @@ class PyTeXBuilder:
             pytex_config: Optional[Union[PyTeXConfig, Path, str]] = None,
             root_dir: Optional[Path] = None
     ):
-        self._build_target_type: Optional[PyTeXRootDirType] = None
         if isinstance(pytex_config, Path) or isinstance(pytex_config, str):
             config_file = Path(pytex_config)
             self._pytex_config: Optional[PyTeXConfig] = PyTeXConfig.from_yaml(config_file)
@@ -31,8 +31,12 @@ class PyTeXBuilder:
             self._root_dir = root_dir
 
         # Non-public attributes
-        self._version_info: Optional[VersionInfo] = None
-        self._pytex_files: Optional[List[PyTeXSourceFile]] = None
+        self._build_target_type: Optional[PyTeXRootDirType] = None
+        self._old_version_info: Optional[VersionInfo] = None
+        self._modified_old_version_info: VersionInfo = VersionInfo()
+        self._pytex_files: List[PyTeXSourceFile] = []
+        self._files_to_clean: List[RelativePath] = []
+        self._files_to_overwrite: List[RelativePath] = []
 
     def build_tex_sources(self) -> bool:
         self._build_target_type = PyTeXRootDirType.TEX_SOURCE
@@ -52,16 +56,19 @@ class PyTeXBuilder:
         return self._build()
 
     @property
-    def version_info(self) -> VersionInfo:
-        if self._version_info is None:
-            version_info_file = self.target_root / VERSION_INFO_FILE
-            if version_info_file.exists():
-                self._version_info = VersionInfo.from_json(
-                    self.target_root / VERSION_INFO_FILE
-                )
-            else:
-                self._version_info = VersionInfo()
-        return self._version_info
+    def old_version_info(self) -> VersionInfo:
+        if self._old_version_info is None:
+            self._old_version_info = self._get_version_info()
+        return self._old_version_info
+
+    def _get_version_info(self) -> VersionInfo:
+        version_info_file = self.target_root / VERSION_INFO_FILE
+        if version_info_file.exists():
+            return VersionInfo.from_json(
+                self.target_root / VERSION_INFO_FILE
+            )
+        else:
+            return VersionInfo()
 
     @property
     def pytex_config(self) -> PyTeXConfig:
@@ -72,6 +79,8 @@ class PyTeXBuilder:
 
     @property
     def target_root(self) -> Path:
+        if self._build_target_type is None:
+            raise NotImplementedError
         return {
             PyTeXRootDirType.BUILD: self.pytex_config.build_dir_specification.build_root,
             PyTeXRootDirType.DOC: self.pytex_config.build_dir_specification.doc_root,
@@ -134,8 +143,50 @@ class PyTeXBuilder:
 
         #  TODO: give pytex source file some additional building information
 
+    def _old_version_lookup(self, relative_path: RelativePath) -> FileVersionInfo:
+        matches = [
+            file_version_info
+            for file_version_info in self.old_version_info.files
+            if file_version_info.relative_name == str(relative_path.relative_path)
+        ]
+        if len(matches) >= 2:
+            raise NotImplementedError
+        elif len(matches) == 1:
+            return matches[0]
+        else:
+            return FileVersionInfo()
+
+    def _check_output_directory_integrity(self):
+        out_dir_files = [
+            RelativePath(self.target_root, file)
+            for file in self.target_root.rglob('*')
+        ]
+        for file in out_dir_files:
+            version = self._old_version_lookup(file)
+            if version.file_hash != md5(file.path):
+                if self.pytex_config.clean_old_files:
+                    raise NotImplementedError  # Not ok
+                else:
+                    if self.pytex_config.overwrite_existing_files:
+                        self._files_to_overwrite.append(file)
+                    else:
+                        pass
+                        # Not ok iff we are going to write this file
+
+    def _update_old_version_info(self):
+        self._modified_old_version_info.files = []  # Make sure this is set so we can get references on it
+        for source_file in self._pytex_files:
+            for output_file in source_file.output_files:
+                self._modified_old_version_info.files.append(
+                    self._old_version_lookup(
+                        output_file
+                    )
+                )
+
     def _build(self) -> bool:
         logger.info("Starting build")
         self._load_pytex_files()
+        logger.info(f"Found {len(self._pytex_files)} source files")
+        self._update_old_version_info()
         return True
 
diff --git a/PyTeX/format/dtx_formatter.py b/PyTeX/format/dtx_formatter.py
index fc635d5..deb02fb 100644
--- a/PyTeX/format/dtx_formatter.py
+++ b/PyTeX/format/dtx_formatter.py
@@ -5,14 +5,16 @@ from .formatting_config import FormattingConfig
 
 
 class DTXFormatter(TexFormatter):
+    @property
     def dependencies(self) -> List[str]:
-        pass
+        return []  # TODO
 
     def future_config(self) -> List[Tuple[str, FormattingConfig]]:
-        pass
+        return []  # TODO
 
+    @property
     def output_files(self) -> List[str]:
-        pass
+        return []  # TODO
 
     def open_output_stream(self, build_dir: Path) -> None:
         pass
diff --git a/main.py b/main.py
index c8e423a..f57ef1b 100644
--- a/main.py
+++ b/main.py
@@ -9,8 +9,6 @@ builder = PyTeXBuilder(conf_path)
 
 builder.build_tex_sources()
 
-v = builder.version_info
-
 
 
 pass