I just found this JKR piece on Extension charms on Pottermore.  Doesn't directly address houses, but it does say the charm's use is regulated: <a rel="nofollow" target="_blank" href="https://www.pottermore.com/writing-by-jk-rowling/extension-charms">Pottermore - Extension Charms</a><div style="width:450px;font-family:'Helvetica Neue', 'Helvetica', 'Arial', 'san-serif', 'Roboto';" id="enhancrCard_1" class="link-enhancr-attachment link-enhancr-element"> <table class="link-enhancr-element" cellspacing="0" cellpadding="0" style="width:450px;height:170px;margin-top:5px;margin-bottom:5px;display:block;"> <tbody><tr class="link-enhancr-element"> <td colspan="6" class="link-enhancr-element" style="height:1px;background-color:#e5e5e5;"> <div class="link-enhancr-element" style="height:1px;background-color:#e5e5e5;"></div> </td> </tr> <tr class="link-enhancr-element"> <td rowspan="2" class="link-enhancr-element" style="width:1px;background-color:#e5e5e5;"> <div class="link-enhancr-element" style="width:1px;background-color:#e5e5e5;"></div> </td>  <td class="link-enhancr-element" rowspan="2" style="vertical-align:middle;width:168px;height:168px;background-color:#000000;"> <div class="link-enhancr-element" style="width:168px;" align="center"> <a rel="nofollow" target="_blank" href="https://www.pottermore.com/writing-by-jk-rowling/extension-charms" class="link-enhancr-card-urlWrapper link-enhancr-element" style="text-decoration:none;color:#000000;"> <img alt="image" src="https://images.pottermore.com/bxd3o8b291gf/4zqWKV8KN2aO2C86IcakOk/c759de793facca444ef046656d541d82/Tent_PM_B7C14M1_TentInWoods_Moment.jpg?w=1200" class="link-enhancr-thumbnail-image link-enhancr-element" width="168" height="66" style="display:block;margin:auto;"> </a> </div> </td> <td rowspan="2" class="link-enhancr-element" style="width:1px;background-color:#e5e5e5;"> <div class="link-enhancr-element" style="width:1px;background-color:#e5e5e5;"></div> </td>  <td class="link-enhancr-element" colspan="2" style="width:100%;vertical-align:middle;font-family:'Helvetica Neue', 'Helvetica', 'Arial', 'san-serif', 'Roboto';"> <div class="link-enhancr-text-part link-enhancr-element" style="line-height:16.5px;background-color:#ffffff;height:130px;width:279px;"> <div class="link-enhancr-element" style="word-wrap:break-word;padding:7px 20px 0px 14px;"> <span class="link-enhancr-element icon icon-shrink link-enhancr-toggle"></span> <span class="link-enhancr-element icon icon-close link-enhancr-delete"></span> <a rel="nofollow" target="_blank" href="https://www.pottermore.com/writing-by-jk-rowling/extension-charms" class="link-enhancr-card-urlWrapper link-enhancr-element" style="text-decoration:none;color:#000000;"> <span class="link-enhancr-element link-enhancr-card-title" style="margin:0;font-weight:normal;margin-bottom:4px;font-size:18px;line-height:21px;max-height:43px;color:#000000;overflow:hidden;display:inline-block;">Pottermore - Extension Charms</span> </a>  <div style="font-size:13px;line-height:20px;color:#999999;max-height:81px;font-family:'Helvetica Neue', 'Helvetica', 'Arial', 'san-serif', 'Roboto';overflow:hidden;" class="link-enhancr-card-description link-enhancr-element">Writing by J.K. Rowling about Extension Charms</div>  </div> </div> </td> <td class="link-enhancr-element" rowspan="2" style="width:1px;background-color:#e5e5e5;"> <div class="link-enhancr-element" style="width:1px;background-color:#e5e5e5;"></div> </td> </tr><tr> <td class="link-enhancr-element" style="width:100%;vertical-align:middle;font-family:'Helvetica Neue', 'Helvetica', 'Arial', 'san-serif', 'Roboto';"> <div class="link-enhancr-element" style="font-size:0pt;padding:7px 20px 9px 15px;"> <a rel="nofollow" target="_blank" href="https://www.pottermore.com/writing-by-jk-rowling/extension-charms" class="link-enhancr-card-url link-enhancr-element" style="color:black;text-decoration:none;cursor:pointer;"> <span class="link-enhancr-element link-enhancr-view-on" style="display:inline-block;line-height:11px;max-width:179px;min-width:119px;overflow:hidden;max-height:13px;"> <span class="link-enhancr-element link-enhancr-mobile-no-resize" style="vertical-align:middle;font-size:9px;line-height:11px;color:#999999;"> View on <span style="font-weight:bold;">www.pottermore.com</span> </span> </span> </a>  </div> </td> <td class="link-enhancr-element" style="vertical-align:middle;width:100px;font-family:'Helvetica Neue', 'Helvetica', 'Arial', 'san-serif', 'Roboto';"> <div class="link-enhancr-element link-enhancr-preview-wrapper" style="padding:9px 20px 12px 0px;max-width:100px;min-width:80px;overflow:hidden;text-align:right;line-height:11px;max-height:13px;font-size:0pt;"> <span class="link-enhancr-element link-enhancr-preview-by link-enhancr-mobile-no-resize" style="line-height:11px;vertical-align:middle;font-size:9px;line-height:11px;color:#999999;"> Preview by Yahoo </span> </div> </td> </tr>  <tr class="link-enhancr-element"> <td class="link-enhancr-element" colspan="6" style="height:1px;background-color:#e5e5e5;"> <div class="link-enhancr-element" style="height:1px;background-color:#e5e5e5;"></div> </td> </tr> </tbody></table> </div><p>  </p>