diff --git a/testing/btest/btest.cfg b/testing/btest/btest.cfg
index ad2ac12d6b..68e4b7ef57 100644
--- a/testing/btest/btest.cfg
+++ b/testing/btest/btest.cfg
@@ -1,3 +1,8 @@
+[DEFAULT]
+# This should be a path relative to the main repository directory, and
+# can be overridden with btest's -s command-line argument.
+build_dir = build
+
 [btest]
 TestDirs    = doc bifs language core scripts coverage signatures plugins broker spicy supervisor telemetry
 TmpDir      = %(testbase)s/.tmp
@@ -7,19 +12,19 @@ IgnoreFiles = *.tmp *.swp .clang-format #* *.trace .DS_Store
 MinVersion  = 0.63
 
 [environment]
-ZEEKPATH=`bash -c %(testbase)s/../../build/zeek-path-dev`
+ZEEKPATH=`bash -c %(testbase)s/../../%(build_dir)s/zeek-path-dev`
 ZEEK_SEED_FILE=%(testbase)s/random.seed
 ZEEK_PLUGIN_PATH=
 TZ=UTC
 LC_ALL=C
 BTEST_PATH=%(testbase)s/../../auxil/btest
-PATH=%(testbase)s/../../build/src:%(testbase)s/../scripts:%(testbase)s/../../auxil/btest:%(testbase)s/../../build/auxil/zeek-aux/zeek-cut:%(testbase)s/../../auxil/btest/sphinx:%(default_path)s:/sbin
+PATH=%(testbase)s/../../%(build_dir)s/src:%(testbase)s/../scripts:%(testbase)s/../../auxil/btest:%(testbase)s/../../%(build_dir)s/auxil/zeek-aux/zeek-cut:%(testbase)s/../../auxil/btest/sphinx:%(default_path)s:/sbin
 TRACES=%(testbase)s/Traces
 FILES=%(testbase)s/Files
 SCRIPTS=%(testbase)s/../scripts
 DOC_ROOT=%(testbase)s/../../doc
 DIST=%(testbase)s/../..
-BUILD=%(testbase)s/../../build
+BUILD=%(testbase)s/../../%(build_dir)s
 TEST_DIFF_CANONIFIER=%(testbase)s/../scripts/diff-canonifier
 TMPDIR=%(testbase)s/.tmp
 ZEEK_PROFILER_FILE=%(testbase)s/.tmp/script-coverage/XXXXXX
@@ -32,8 +37,8 @@ ZEEK_DISABLE_ZEEKYGEN=1
 ZEEK_ALLOW_INIT_ERRORS=1
 ZEEK_SUPERVISOR_NO_SIGKILL=1
 UBSAN_OPTIONS=print_stacktrace=1
-SPICY_PATH=`bash -c %(testbase)s/../../build/spicy-path`
-HILTI_CXX_INCLUDE_DIRS=`bash -c %(testbase)s/../../build/hilti-cxx-include-dirs`
+SPICY_PATH=`bash -c %(testbase)s/../../%(build_dir)s/spicy-path`
+HILTI_CXX_INCLUDE_DIRS=`bash -c %(testbase)s/../../%(build_dir)s/hilti-cxx-include-dirs`
 
 [environment-AST-dup]
 # Environment for testing AST duplication functionality, which is