diff --git a/carddav/server.go b/carddav/server.go
index d4cf876..5aeb714 100644
--- a/carddav/server.go
+++ b/carddav/server.go
@@ -439,3 +439,28 @@ func (b *backend) Copy(r *http.Request, dest *internal.Href, recursive, overwrit
 func (b *backend) Move(r *http.Request, dest *internal.Href, overwrite bool) (created bool, err error) {
 	panic("TODO")
 }
+
+// https://tools.ietf.org/rfcmarkup?doc=6352#section-6.3.2.1
+type PreconditionType string
+
+const (
+	PreconditionNoUIDConflict        PreconditionType = "no-uid-conflict"
+	PreconditionSupportedAddressData PreconditionType = "supported-address-data"
+	PreconditionValidAddressData     PreconditionType = "valid-address-data"
+	PreconditionMaxResourceSize      PreconditionType = "max-resource-size"
+)
+
+func NewPreconditionError(err PreconditionType) error {
+	name := xml.Name{"urn:ietf:params:xml:ns:carddav", string(err)}
+	elem := internal.NewRawXMLElement(name, nil, nil)
+	e := internal.Error{
+		Raw: []internal.RawXMLValue{
+			*elem,
+		},
+	}
+	return &internal.DAVError{
+		Code: 409,
+		Msg:  fmt.Sprintf("precondition not met: %s", string(err)),
+		Err:  e,
+	}
+}
diff --git a/internal/internal.go b/internal/internal.go
index 08c2ef8..065e66f 100644
--- a/internal/internal.go
+++ b/internal/internal.go
@@ -99,3 +99,14 @@ func (err *HTTPError) Error() string {
 		return s
 	}
 }
+
+// DAVError is a XML error with HTTP status and a human readable message
+type DAVError struct {
+	Code int
+	Msg  string
+	Err  Error
+}
+
+func (err *DAVError) Error() string {
+	return err.Msg
+}
diff --git a/internal/server.go b/internal/server.go
index 5a51aed..66f4964 100644
--- a/internal/server.go
+++ b/internal/server.go
@@ -10,6 +10,12 @@ import (
 )
 
 func ServeError(w http.ResponseWriter, err error) {
+	if davErr, ok := err.(*DAVError); ok {
+		w.WriteHeader(davErr.Code)
+		ServeXML(w).Encode(davErr.Err)
+		return
+	}
+
 	code := http.StatusInternalServerError
 	if httpErr, ok := err.(*HTTPError); ok {
 		code = httpErr.Code
@@ -102,11 +108,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if err != nil {
-		code := http.StatusInternalServerError
-		if httpErr, ok := err.(*HTTPError); ok {
-			code = httpErr.Code
-		}
-		http.Error(w, err.Error(), code)
+		ServeError(w, err)
 	}
 }