diff --git a/docs/changelog/1575.feature.rst b/docs/changelog/1575.feature.rst
new file mode 100644
index 00000000..7a36ab20
--- /dev/null
+++ b/docs/changelog/1575.feature.rst
@@ -0,0 +1,3 @@
+Implement support for building projects
+having :pep:`517#in-tree-build-backends` ``backend-path`` setting -
+by :user:`webknjaz`
diff --git a/src/tox/helper/build_isolated.py b/src/tox/helper/build_isolated.py
index 55ea41b8..3d897097 100644
--- a/src/tox/helper/build_isolated.py
+++ b/src/tox/helper/build_isolated.py
@@ -1,10 +1,40 @@
+"""PEP 517 build backend invocation script.
+
+It accepts externally parsed build configuration from `[build-system]`
+in `pyproject.toml` and invokes an API endpoint for building an sdist
+tarball.
+"""
+
+import os
 import sys
 
+
+def _ensure_module_in_paths(module, paths):
+    """Verify that the imported backend belongs in-tree."""
+    if not paths:
+        return
+
+    module_path = os.path.normcase(os.path.abspath(module.__file__))
+    normalized_paths = (os.path.normcase(os.path.abspath(path)) for path in paths)
+
+    if any(os.path.commonprefix((module_path, path)) == path for path in normalized_paths):
+        return
+
+    raise SystemExit(
+        "build-backend ({!r}) must exist in one of the paths "
+        "specified by backend-path ({!r})".format(module, paths),
+    )
+
+
 dist_folder = sys.argv[1]
 backend_spec = sys.argv[2]
 backend_obj = sys.argv[3] if len(sys.argv) >= 4 else None
+backend_paths = sys.argv[4].split(os.path.pathsep) if sys.argv[4] else []
+
+sys.path[:0] = backend_paths
 
 backend = __import__(backend_spec, fromlist=["_trash"])
+_ensure_module_in_paths(backend, backend_paths)
 if backend_obj:
     backend = getattr(backend, backend_obj)
 
diff --git a/src/tox/package/builder/isolated.py b/src/tox/package/builder/isolated.py
index c02aa109..38ebe8ea 100644
--- a/src/tox/package/builder/isolated.py
+++ b/src/tox/package/builder/isolated.py
@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import json
+import os
 from collections import namedtuple
 
 import six
@@ -11,7 +12,9 @@ from tox import reporter
 from tox.config import DepConfig, get_py_project_toml
 from tox.constants import BUILD_ISOLATED, BUILD_REQUIRE_SCRIPT
 
-BuildInfo = namedtuple("BuildInfo", ["requires", "backend_module", "backend_object"])
+BuildInfo = namedtuple(
+    "BuildInfo", ["requires", "backend_module", "backend_object", "backend_paths"],
+)
 
 
 def build(config, session):
@@ -84,7 +87,12 @@ def get_build_info(folder):
     module = args[0]
     obj = args[1] if len(args) > 1 else ""
 
-    return BuildInfo(requires, module, obj)
+    backend_paths = build_system.get("backend-path", [])
+    if not isinstance(backend_paths, list):
+        abort("backend-path key at build-system section must be a list, if specified")
+    backend_paths = [folder.join(p) for p in backend_paths]
+
+    return BuildInfo(requires, module, obj, backend_paths)
 
 
 def perform_isolated_build(build_info, package_venv, dist_dir, setup_dir):
@@ -103,6 +111,7 @@ def perform_isolated_build(build_info, package_venv, dist_dir, setup_dir):
                 str(dist_dir),
                 build_info.backend_module,
                 build_info.backend_object,
+                os.path.pathsep.join(str(p) for p in build_info.backend_paths),
             ],
             returnout=True,
             action=action,

diff --git a/tests/unit/package/builder/test_package_builder_isolated.py b/tests/unit/package/builder/test_package_builder_isolated.py
index d43ed5f1..63e0685a 100644
--- a/tests/unit/package/builder/test_package_builder_isolated.py
+++ b/tests/unit/package/builder/test_package_builder_isolated.py
@@ -138,3 +138,18 @@ def test_package_isolated_toml_bad_backend(initproj):
     build-backend = []
     """,
     )
+
+
+def test_package_isolated_toml_bad_backend_path(initproj):
+    """Verify that a non-list 'backend-path' is forbidden."""
+    toml_file_check(
+        initproj,
+        6,
+        "backend-path key at build-system section must be a list, if specified",
+        """
+    [build-system]
+    requires = []
+    build-backend = 'setuptools.build_meta'
+    backend-path = 42
+    """,
+    )

Support PEP517 in-tree build backend
I'm getting
```python-traceback
ERROR: invocation failed (exit code 1), logfile: ~/src/github/ansible/pylibssh/.tox/.package/log/.package-2.log
=================================== log start ====================================
Traceback (most recent call last):
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/tox/helper/build_requires.py", line 7, in <module>
    backend = __import__(backend_spec, fromlist=[None])
ModuleNotFoundError: No module named 'pep517_backend'
```
because of
```console
$ cat pyproject.toml                            
[build-system]
...
backend-path = ["bin"]  # requires 'Pip>=20' or 'pep517>=0.6.0'
build-backend = "pep517_backend"
```
and
```console
$ ls -l bin/pep517_backend.py      
-rw-r--r-- 1 me me 6.2K May  5 00:43 bin/pep517_backend.py
```

I think this should be considered a bug. Pip supports it starting v20 and pep517 supports it with v0.7 or so.
tests/unit/package/builder/test_package_builder_isolated.py::test_package_isolated_toml_bad_backend_path
tests/unit/package/builder/test_package_builder_isolated.py::test_package_isolated_toml_no_backend tests/unit/package/builder/test_package_builder_isolated.py::test_package_isolated_toml_bad_requires tests/unit/package/builder/test_package_builder_isolated.py::test_package_isolated_toml_bad_backend tests/unit/package/builder/test_package_builder_isolated.py::test_package_isolated_toml_no_requires tests/unit/package/builder/test_package_builder_isolated.py::test_package_isolated_toml_no_build_system