diff --git a/src/network/network.cpp b/src/network/network.cpp
index de11f48eef..321e47ffb7 100644
--- a/src/network/network.cpp
+++ b/src/network/network.cpp
@@ -37,6 +37,7 @@
 #include "../gfx_func.h"
 #include "../error.h"
 #include "../misc_cmd.h"
+#include "../core/string_builder.hpp"
 #ifdef DEBUG_DUMP_COMMANDS
 #	include "../fileio_func.h"
 #endif
@@ -231,49 +232,48 @@ uint8_t NetworkSpectatorCount()
  * If 'self_send' is true, this is the client who is sending the message */
 void NetworkTextMessage(NetworkAction action, TextColour colour, bool self_send, const std::string &name, const std::string &str, StringParameter &&data)
 {
-	std::string string;
-	switch (action) {
-		case NETWORK_ACTION_SERVER_MESSAGE:
-			/* Ignore invalid messages */
-			string = GetString(STR_NETWORK_SERVER_MESSAGE, str);
-			colour = CC_DEFAULT;
-			break;
-		case NETWORK_ACTION_COMPANY_SPECTATOR:
-			colour = CC_DEFAULT;
-			string = GetString(STR_NETWORK_MESSAGE_CLIENT_COMPANY_SPECTATE, name);
-			break;
-		case NETWORK_ACTION_COMPANY_JOIN:
-			colour = CC_DEFAULT;
-			string = GetString(STR_NETWORK_MESSAGE_CLIENT_COMPANY_JOIN, name, str);
-			break;
-		case NETWORK_ACTION_COMPANY_NEW:
-			colour = CC_DEFAULT;
-			string = GetString(STR_NETWORK_MESSAGE_CLIENT_COMPANY_NEW, name, std::move(data));
-			break;
-		case NETWORK_ACTION_JOIN:
-			/* Show the Client ID for the server but not for the client. */
-			string = _network_server ?
-					GetString(STR_NETWORK_MESSAGE_CLIENT_JOINED_ID, name, std::move(data)) :
-					GetString(STR_NETWORK_MESSAGE_CLIENT_JOINED, name);
-			break;
-		case NETWORK_ACTION_LEAVE:          string = GetString(STR_NETWORK_MESSAGE_CLIENT_LEFT, name, std::move(data)); break;
-		case NETWORK_ACTION_NAME_CHANGE:    string = GetString(STR_NETWORK_MESSAGE_NAME_CHANGE, name, str); break;
-		case NETWORK_ACTION_GIVE_MONEY:     string = GetString(STR_NETWORK_MESSAGE_GIVE_MONEY, name, std::move(data), str); break;
-		case NETWORK_ACTION_KICKED:         string = GetString(STR_NETWORK_MESSAGE_KICKED, name, str); break;
-		case NETWORK_ACTION_CHAT_COMPANY:   string = GetString(self_send ? STR_NETWORK_CHAT_TO_COMPANY : STR_NETWORK_CHAT_COMPANY, name, str); break;
-		case NETWORK_ACTION_CHAT_CLIENT:    string = GetString(self_send ? STR_NETWORK_CHAT_TO_CLIENT  : STR_NETWORK_CHAT_CLIENT, name, str);  break;
-		case NETWORK_ACTION_EXTERNAL_CHAT:  string = GetString(STR_NETWORK_CHAT_EXTERNAL, std::move(data), name, str); break;
-		default:                            string = GetString(STR_NETWORK_CHAT_ALL, name, str); break;
-	}
+	std::string message;
+	StringBuilder builder(message);
 
 	/* All of these strings start with "***". These characters are interpreted as both left-to-right and
 	 * right-to-left characters depending on the context. As the next text might be an user's name, the
 	 * user name's characters will influence the direction of the "***" instead of the language setting
 	 * of the game. Manually set the direction of the "***" by inserting a text-direction marker. */
-	std::ostringstream stream;
-	std::ostreambuf_iterator<char> iterator(stream);
-	Utf8Encode(iterator, _current_text_dir == TD_LTR ? CHAR_TD_LRM : CHAR_TD_RLM);
-	std::string message = stream.str() + string;
+	builder.PutUtf8(_current_text_dir == TD_LTR ? CHAR_TD_LRM : CHAR_TD_RLM);
+
+	switch (action) {
+		case NETWORK_ACTION_SERVER_MESSAGE:
+			/* Ignore invalid messages */
+			builder += GetString(STR_NETWORK_SERVER_MESSAGE, str);
+			colour = CC_DEFAULT;
+			break;
+		case NETWORK_ACTION_COMPANY_SPECTATOR:
+			colour = CC_DEFAULT;
+			builder += GetString(STR_NETWORK_MESSAGE_CLIENT_COMPANY_SPECTATE, name);
+			break;
+		case NETWORK_ACTION_COMPANY_JOIN:
+			colour = CC_DEFAULT;
+			builder += GetString(STR_NETWORK_MESSAGE_CLIENT_COMPANY_JOIN, name, str);
+			break;
+		case NETWORK_ACTION_COMPANY_NEW:
+			colour = CC_DEFAULT;
+			builder += GetString(STR_NETWORK_MESSAGE_CLIENT_COMPANY_NEW, name, std::move(data));
+			break;
+		case NETWORK_ACTION_JOIN:
+			/* Show the Client ID for the server but not for the client. */
+			builder += _network_server ?
+					GetString(STR_NETWORK_MESSAGE_CLIENT_JOINED_ID, name, std::move(data)) :
+					GetString(STR_NETWORK_MESSAGE_CLIENT_JOINED, name);
+			break;
+		case NETWORK_ACTION_LEAVE:          builder += GetString(STR_NETWORK_MESSAGE_CLIENT_LEFT, name, std::move(data)); break;
+		case NETWORK_ACTION_NAME_CHANGE:    builder += GetString(STR_NETWORK_MESSAGE_NAME_CHANGE, name, str); break;
+		case NETWORK_ACTION_GIVE_MONEY:     builder += GetString(STR_NETWORK_MESSAGE_GIVE_MONEY, name, std::move(data), str); break;
+		case NETWORK_ACTION_KICKED:         builder += GetString(STR_NETWORK_MESSAGE_KICKED, name, str); break;
+		case NETWORK_ACTION_CHAT_COMPANY:   builder += GetString(self_send ? STR_NETWORK_CHAT_TO_COMPANY : STR_NETWORK_CHAT_COMPANY, name, str); break;
+		case NETWORK_ACTION_CHAT_CLIENT:    builder += GetString(self_send ? STR_NETWORK_CHAT_TO_CLIENT  : STR_NETWORK_CHAT_CLIENT, name, str);  break;
+		case NETWORK_ACTION_EXTERNAL_CHAT:  builder += GetString(STR_NETWORK_CHAT_EXTERNAL, std::move(data), name, str); break;
+		default:                            builder += GetString(STR_NETWORK_CHAT_ALL, name, str); break;
+	}
 
 	Debug(desync, 1, "msg: {:08x}; {:02x}; {}", TimerGameEconomy::date, TimerGameEconomy::date_fract, message);
 	IConsolePrint(colour, message);
diff --git a/src/saveload/strings_sl.cpp b/src/saveload/strings_sl.cpp
index 2be7fe0f4a..94caa07b20 100644
--- a/src/saveload/strings_sl.cpp
+++ b/src/saveload/strings_sl.cpp
@@ -10,6 +10,7 @@
 #include "../stdafx.h"
 #include "../string_func.h"
 #include "../strings_func.h"
+#include "../core/string_builder.hpp"
 #include "saveload_internal.h"
 
 #include "table/strings.h"
@@ -63,8 +64,8 @@ std::string CopyFromOldName(StringID id)
 	if (IsSavegameVersionBefore(SLV_37)) {
 		const std::string &strfrom = _old_name_array[GB(id, 0, 9)];
 
-		std::ostringstream tmp;
-		std::ostreambuf_iterator<char> strto(tmp);
+		std::string result;
+		StringBuilder builder(result);
 		for (char s : strfrom) {
 			if (s == '\0') break;
 			char32_t c = static_cast<uint8_t>(s); // cast to unsigned before integer promotion
@@ -82,10 +83,10 @@ std::string CopyFromOldName(StringID id)
 				default: break;
 			}
 
-			if (IsPrintable(c)) Utf8Encode(strto, c);
+			if (IsPrintable(c)) builder.PutUtf8(c);
 		}
 
-		return tmp.str();
+		return result;
 	} else {
 		/* Name will already be in UTF-8. */
 		return StrMakeValid(_old_name_array[GB(id, 0, 9)]);
diff --git a/src/strings.cpp b/src/strings.cpp
index e6ab3fc047..68a59c0c9b 100644
--- a/src/strings.cpp
+++ b/src/strings.cpp
@@ -2371,13 +2371,12 @@ void CheckForMissingGlyphs(bool base_font, MissingGlyphSearcher *searcher)
 		if (!bad_font && any_font_configured) {
 			/* If the user configured a bad font, and we found a better one,
 			 * show that we loaded the better font instead of the configured one.
-			 * The colour 'character' might change in the
-			 * future, so for safety we just Utf8 Encode it into the string,
-			 * which takes exactly three characters, so it replaces the "XXX"
-			 * with the colour marker. */
-			static std::string err_str("XXXThe current font is missing some of the characters used in the texts for this language. Using system fallback font instead.");
-			Utf8Encode(err_str.data(), SCC_YELLOW);
-			ShowErrorMessage(GetEncodedString(STR_JUST_RAW_STRING, err_str), {}, WL_WARNING);
+			 */
+			std::string err_str;
+			StringBuilder builder(err_str);
+			builder.PutUtf8(SCC_YELLOW);
+			builder.Put("The current font is missing some of the characters used in the texts for this language. Using system fallback font instead.");
+			ShowErrorMessage(GetEncodedString(STR_JUST_RAW_STRING, std::move(err_str)), {}, WL_WARNING);
 		}
 
 		if (bad_font && base_font) {
@@ -2393,11 +2392,12 @@ void CheckForMissingGlyphs(bool base_font, MissingGlyphSearcher *searcher)
 		/* All attempts have failed. Display an error. As we do not want the string to be translated by
 		 * the translators, we 'force' it into the binary and 'load' it via a BindCString. To do this
 		 * properly we have to set the colour of the string, otherwise we end up with a lot of artifacts.
-		 * The colour 'character' might change in the future, so for safety we just Utf8 Encode it into
-		 * the string, which takes exactly three characters, so it replaces the "XXX" with the colour marker. */
-		static std::string err_str("XXXThe current font is missing some of the characters used in the texts for this language. Go to Help & Manuals > Fonts, or read the file docs/fonts.md in your OpenTTD directory, to see how to solve this.");
-		Utf8Encode(err_str.data(), SCC_YELLOW);
-		ShowErrorMessage(GetEncodedString(STR_JUST_RAW_STRING, err_str), {}, WL_WARNING);
+		 */
+		std::string err_str;
+		StringBuilder builder(err_str);
+		builder.PutUtf8(SCC_YELLOW);
+		builder.Put("The current font is missing some of the characters used in the texts for this language. Go to Help & Manuals > Fonts, or read the file docs/fonts.md in your OpenTTD directory, to see how to solve this.");
+		ShowErrorMessage(GetEncodedString(STR_JUST_RAW_STRING, std::move(err_str)), {}, WL_WARNING);
 
 		/* Reset the font width */
 		LoadStringWidthTable(searcher->Monospace());
@@ -2415,16 +2415,14 @@ void CheckForMissingGlyphs(bool base_font, MissingGlyphSearcher *searcher)
 	 * be translated by the translators, we 'force' it into the
 	 * binary and 'load' it via a BindCString. To do this
 	 * properly we have to set the colour of the string,
-	 * otherwise we end up with a lot of artifacts. The colour
-	 * 'character' might change in the future, so for safety
-	 * we just Utf8 Encode it into the string, which takes
-	 * exactly three characters, so it replaces the "XXX" with
-	 * the colour marker.
+	 * otherwise we end up with a lot of artifacts.
 	 */
 	if (_current_text_dir != TD_LTR) {
-		static std::string err_str("XXXThis version of OpenTTD does not support right-to-left languages. Recompile with ICU + Harfbuzz enabled.");
-		Utf8Encode(err_str.data(), SCC_YELLOW);
-		ShowErrorMessage(GetEncodedString(STR_JUST_RAW_STRING, err_str), {}, WL_ERROR);
+		std::string err_str;
+		StringBuilder builder(err_str);
+		builder.PutUtf8(SCC_YELLOW);
+		builder.Put("This version of OpenTTD does not support right-to-left languages. Recompile with ICU + Harfbuzz enabled.");
+		ShowErrorMessage(GetEncodedString(STR_JUST_RAW_STRING, std::move(err_str)), {}, WL_ERROR);
 	}
 #endif /* !(WITH_ICU_I18N && WITH_HARFBUZZ) && !WITH_UNISCRIBE && !WITH_COCOA */
 }
diff --git a/src/tests/string_func.cpp b/src/tests/string_func.cpp
index 93853ed795..1b05786a9e 100644
--- a/src/tests/string_func.cpp
+++ b/src/tests/string_func.cpp
@@ -13,6 +13,7 @@
 
 #include "../string_func.h"
 #include "../strings_func.h"
+#include "../core/string_builder.hpp"
 #include "../table/control_codes.h"
 
 #include "table/strings.h"
@@ -423,15 +424,15 @@ static std::string FixSCCEncodedWrapper(const std::string &str, bool fix_code)
 }
 
 /* Helper to compose a string part from a unicode character */
-static void ComposePart(std::back_insert_iterator<std::string> &output, char32_t c)
+static void ComposePart(StringBuilder &builder, char32_t c)
 {
-	Utf8Encode(output, c);
+	builder.PutUtf8(c);
 }
 
 /* Helper to compose a string part from a string. */
-static void ComposePart(std::back_insert_iterator<std::string> &output, const std::string &value)
+static void ComposePart(StringBuilder &builder, const std::string &value)
 {
-	for (const auto &c : value) *output = c;
+	builder += value;
 }
 
 /* Helper to compose a string from unicode or string parts. */
@@ -439,8 +440,8 @@ template <typename... Args>
 static std::string Compose(Args &&... args)
 {
 	std::string result;
-	auto output = std::back_inserter(result);
-	(ComposePart(output, args), ...);
+	StringBuilder builder(result);
+	(ComposePart(builder, args), ...);
 	return result;
 }
 
diff --git a/src/textfile_gui.cpp b/src/textfile_gui.cpp
index 82e06a02b2..0ac1f3a3da 100644
--- a/src/textfile_gui.cpp
+++ b/src/textfile_gui.cpp
@@ -14,6 +14,7 @@
 #include "gfx_type.h"
 #include "gfx_func.h"
 #include "string_func.h"
+#include "core/string_builder.hpp"
 #include "textfile_gui.h"
 #include "dropdown_type.h"
 #include "dropdown_func.h"
@@ -245,7 +246,7 @@ void TextfileWindow::FindHyperlinksInMarkdown(Line &line, size_t line_index)
 {
 	std::string::const_iterator last_match_end = line.text.cbegin();
 	std::string fixed_line;
-	char ccbuf[5];
+	StringBuilder builder(fixed_line);
 
 	std::sregex_iterator matcher{ line.text.cbegin(), line.text.cend(), _markdown_link_regex};
 	while (matcher != std::sregex_iterator()) {
@@ -273,13 +274,13 @@ void TextfileWindow::FindHyperlinksInMarkdown(Line &line, size_t line_index)
 
 		if (link_colour != SCC_CONTROL_END) {
 			/* Format the link to look like a link. */
-			fixed_line += std::string(last_match_end, match[0].first);
+			builder += std::string_view(last_match_end, match[0].first);
 			link.begin = fixed_line.length();
-			fixed_line += std::string(ccbuf, Utf8Encode(ccbuf, SCC_PUSH_COLOUR));
-			fixed_line += std::string(ccbuf, Utf8Encode(ccbuf, link_colour));
-			fixed_line += match[1].str();
+			builder.PutUtf8(SCC_PUSH_COLOUR);
+			builder.PutUtf8(link_colour);
+			builder += match[1].str();
 			link.end = fixed_line.length();
-			fixed_line += std::string(ccbuf, Utf8Encode(ccbuf, SCC_POP_COLOUR));
+			builder.PutUtf8(SCC_POP_COLOUR);
 			last_match_end = match[0].second;
 		}