diff --git a/packages/fastify-vite/config.js b/packages/fastify-vite/config.js
index 4fd4e2e3..769a53eb 100644
--- a/packages/fastify-vite/config.js
+++ b/packages/fastify-vite/config.js
@@ -11,6 +11,7 @@ const {
   stat,
   read,
   sep,
+  determineOutDirRoot,
 } = require('./ioutils.cjs')
 const { createHtmlTemplateFunction } = require('./html.js')
 
@@ -317,25 +318,19 @@ async function resolveViteConfig(root, dev, { spa, distDir } = {}) {
   ]
 }
 
-async function determineOutDirRoot(vite) {
-  const { usePathsRelativeToAppRoot } = vite.fastify
-  if (usePathsRelativeToAppRoot) {
-    const { packageDirectory } = await import('package-directory')
-    return await packageDirectory()
-  }
-  return vite.root
-}
-
 async function resolveSSRBundle({ dev, vite }) {
   const bundle = {}
   let clientOutDir
 
   if (!dev) {
     if (vite.fastify) {
-      clientOutDir = resolveIfRelative(vite.fastify.outDirs.client, await determineOutDirRoot(vite))
+      clientOutDir = await resolveIfRelative(
+        vite.fastify.outDirs.client,
+        await determineOutDirRoot(vite),
+      )
     } else {
       // Backwards compatibility for projects that do not use the viteFastify plugin.
-      bundle.dir = resolveIfRelative(vite.build.outDir, vite.root)
+      bundle.dir = await resolveIfRelative(vite.build.outDir, vite.root)
       clientOutDir = resolve(bundle.dir, 'client')
     }
 
@@ -369,10 +364,13 @@ async function resolveSPABundle({ dev, vite }) {
     let clientOutDir
 
     if (vite.fastify) {
-      clientOutDir = resolveIfRelative(vite.fastify.outDirs.client, await determineOutDirRoot(vite))
+      clientOutDir = await resolveIfRelative(
+        vite.fastify.outDirs.client,
+        await determineOutDirRoot(vite),
+      )
     } else {
       // Backwards compatibility for projects that do not use the viteFastify plugin.
-      bundle.dir = resolveIfRelative(vite.build.outDir, vite.root)
+      bundle.dir = await resolveIfRelative(vite.build.outDir, vite.root)
       clientOutDir = resolve(bundle.dir, 'client')
     }
 
diff --git a/packages/fastify-vite/ioutils.cjs b/packages/fastify-vite/ioutils.cjs
index 9bafec72..a633e4a0 100644
--- a/packages/fastify-vite/ioutils.cjs
+++ b/packages/fastify-vite/ioutils.cjs
@@ -12,8 +12,45 @@ const {
 const { ensureDir, remove } = require('fs-extra')
 const klaw = require('klaw')
 
-function resolveIfRelative(p, root) {
-  return isAbsolute(p) ? p : resolve(root, p)
+function getAppRoot() {
+  const file = require?.main?.filename || process.argv[1]
+  if (!file) return
+
+  return dirname(resolve(file))
+}
+
+async function resolveIfRelative(p, root) {
+  if (isAbsolute(p)) return p
+  if (isAbsolute(root)) return resolve(root, p)
+  if (existsSync(resolve(root, p))) return resolve(root, p)
+
+  const { packageDirectory } = await import('package-directory')
+  let _packageDirectory = await packageDirectory()
+
+  if (!_packageDirectory) {
+    _packageDirectory = await packageDirectory({
+      cwd: getAppRoot() || process.cwd(),
+    })
+  }
+
+  return resolve(_packageDirectory, root, p)
+}
+
+async function determineOutDirRoot(vite) {
+  const { usePathsRelativeToAppRoot } = vite.fastify
+  if (usePathsRelativeToAppRoot) {
+    const { packageDirectory } = await import('package-directory')
+    const _packageDirectory = await packageDirectory()
+    if (_packageDirectory) {
+      return _packageDirectory
+    }
+
+    return await packageDirectory({
+      cwd: getAppRoot() || process.cwd(),
+    })
+  }
+
+  return vite.root
 }
 
 async function* walk(dir, ignorePatterns = []) {
@@ -43,6 +80,8 @@ module.exports = {
   remove,
   isAbsolute,
   sep,
+  getAppRoot,
+  determineOutDirRoot,
   write: writeFile,
   read: readFile,
   exists: existsSync,
diff --git a/packages/fastify-vite/mode/production.js b/packages/fastify-vite/mode/production.js
index 81edf778..fec57200 100644
--- a/packages/fastify-vite/mode/production.js
+++ b/packages/fastify-vite/mode/production.js
@@ -1,6 +1,12 @@
 const { join, resolve } = require('node:path')
 const FastifyStatic = require('@fastify/static')
-const { parse, resolveIfRelative, read, exists } = require('../ioutils.cjs')
+const {
+  parse,
+  resolveIfRelative,
+  read,
+  exists,
+  determineOutDirRoot,
+} = require('../ioutils.cjs')
 
 function fileUrl(str) {
   if (typeof str !== 'string') {
@@ -23,19 +29,15 @@ async function setup(config) {
   let ssrOutDir
 
   if (vite.fastify) {
-    const { outDirs, usePathsRelativeToAppRoot } = vite.fastify
+    const { outDirs } = vite.fastify
 
-    let outDirRoot = vite.root
-    if (usePathsRelativeToAppRoot) {
-      const { packageDirectory } = await import('package-directory')
-      outDirRoot = await packageDirectory()
-    }
+    const outDirRoot = await determineOutDirRoot(vite)
 
-    clientOutDir = resolveIfRelative(outDirs.client, outDirRoot)
-    ssrOutDir = resolveIfRelative(outDirs.ssr || '', outDirRoot)
+    clientOutDir = await resolveIfRelative(outDirs.client, outDirRoot)
+    ssrOutDir = await resolveIfRelative(outDirs.ssr || '', outDirRoot)
   } else {
     // Backwards compatibility for projects that do not use the viteFastify plugin.
-    const outDir = resolveIfRelative(vite.build.outDir, vite.root)
+    const outDir = await resolveIfRelative(vite.build.outDir, vite.root)
 
     clientOutDir = resolve(outDir, 'client')
     ssrOutDir = resolve(outDir, 'server')
diff --git a/packages/fastify-vite/plugin.mjs b/packages/fastify-vite/plugin.mjs
index ebe53c8a..9200e509 100644
--- a/packages/fastify-vite/plugin.mjs
+++ b/packages/fastify-vite/plugin.mjs
@@ -1,8 +1,12 @@
+import { writeFile } from 'node:fs/promises'
 import { isAbsolute, join, relative, sep } from 'node:path'
 import getDeepMergeFunction from '@fastify/deepmerge'
-import { writeFile } from 'node:fs/promises'
 
-export function viteFastify({ spa, clientModule, useRelativePaths = false } = {}) {
+export function viteFastify({
+  spa,
+  clientModule,
+  useRelativePaths = false,
+} = {}) {
   let customOutDir
   let jsonFilePath
   let configToWrite = {}
@@ -30,7 +34,8 @@ export function viteFastify({ spa, clientModule, useRelativePaths = false } = {}
         rawConfig.environments.client ?? {},
       )
       if (!spa) {
-        const ssrEntryPoint = clientModule ?? resolveClientModule(rawConfig.root)
+        const ssrEntryPoint =
+          clientModule ?? resolveClientModule(rawConfig.root)
         rawConfig.environments.ssr = deepMerge(
           createSSREnvironment(isDevMode, outDir, ssrEntryPoint),
           rawConfig.environments.ssr ?? {},
@@ -99,10 +104,11 @@ export function viteFastify({ spa, clientModule, useRelativePaths = false } = {}
       let commonDistFolder = customOutDir // respect custom build.outDir config if provided
       if (!commonDistFolder) {
         const outDirs = Object.values(fastify.outDirs)
-        commonDistFolder = outDirs.length > 1
-          ? findCommonPath(outDirs)
-          // Handle SPA case where there's only dist/client
-          : outDirs[0].split(sep)[0]
+        commonDistFolder =
+          outDirs.length > 1
+            ? findCommonPath(outDirs)
+            : // Handle SPA case where there's only dist/client
+              outDirs[0].split(sep)[0]
       }
 
       if (isAbsolute(commonDistFolder)) {
@@ -152,7 +158,10 @@ async function makeAllPathsRelative(viteConfig) {
   }
 
   Object.keys(fastify.outDirs).forEach((key) => {
-    fastify.outDirs[key] = relative(applicationRootDirectory, fastify.outDirs[key])
+    fastify.outDirs[key] = relative(
+      applicationRootDirectory,
+      fastify.outDirs[key],
+    )
   })
 }