Loft Conversion Calculators

A loft conversion runs through more of the Building Regulations than almost any other domestic project: structure, fire escape, stairs, insulation, sound and electrics all apply, and several trades depend on each other's work being finished first. This page is a working template for the whole job: the task sequence, what you will be ordering, where the regulations touch the work, and the calculator that serves each step.

The two early decisions that matter most are feasibility (is there enough height, and can the floor be made to work?) and the staircase position, because the stairs consume floor area on the storey below and their design is regulated. Settle both, get the structural design done, and the rest of the job follows in a predictable order.

Work in this order

1. 1

   Feasibility before anything else

   The first question is whether the loft can be converted at all. Check the available head height with the Loft Head Height Calculator, whether the existing joists can carry habitable loads with the Loft Load Capacity Calculator, and where a staircase can land with the Loft Staircase Space Calculator. Fix the staircase position early: its geometry and the floor area it takes from the storey below drive the whole layout. Budget with the Loft Conversion Cost Calculator.

2. 2

   Permissions and notices

   Most loft conversions fall within permitted development, but the roof-volume limits depend on your house type: check with the Loft PD Volume Calculator and, for dormers, the Dormer Sizing Calculator. Every conversion needs Building Regulations approval regardless of the planning position: estimate fees with the Building Control Fee Calculator. In a semi or terrace, steelwork often bears on the party wall, so check notice requirements with the Party Wall Checker.

3. 3

   Structural design

   A structural engineer designs the new floor and any steels before work starts; Building Control will expect the calculations. The Loft Steel Beam Calculator and Loft Floor Strengthening Calculator explain what the steels and new joists do and give indicative quantities for budgeting.

4. 4

   Structure in, roof altered, weathertight again

   Steels and the strengthened floor go in first, because every later trade stands on them. Dormers, hip-to-gable or mansard alterations follow, and the roof must be made weathertight again before internal work starts: water getting into the fabric ruins insulation and boarding. Quantify the alteration with the Dormer Sizing, Hip-to-Gable or Mansard Loft Calculator, with the Lead Flashing Calculator for the junctions.

5. 5

   First fix while the walls are open

   Cables and pipes go in before any insulation or boarding: once the boards are up, routing services is much harder. Plan circuits with the Loft Electrical Calculator, and if the conversion includes an en-suite, plan the soil pipe route and pump requirements early with the Loft Bathroom Calculator: waste routes are much harder to add later than cables.

6. 6

   Insulation without blocking the roof ventilation

   Rafter-level insulation has to meet Part L: size it with the Loft Conversion Insulation Calculator and check the build-up with the U-Value Calculator. A warmer, better-sealed roof still needs clear ventilation paths so the structure stays dry: check them with the Loft Ventilation Calculator before the insulation goes in.

7. 7

   Board, skim and upgrade the party wall

   Plasterboard follows first fix, and skim follows the boards: the Plasterboard Calculator and Skim Coat Calculator cover the quantities. In a semi or terrace, the party wall usually needs sound and fire boarding at the same stage: the Loft Party Wall Calculator quantifies it against Part E and Part B.

8. 8

   Stairs, fire safety and finishing

   A habitable loft conversion normally needs a proper fixed staircase, and its rise, going, pitch and headroom are governed by Part K: check the design with the Part K Stair Calculator. Adding a storey also changes the fire-escape picture for the whole house (protected stairway, fire doors, mains smoke alarms): the Extension Fire Safety Calculator covers the Part B requirements for loft conversions. Then flooring, skirting and decoration finish the job.

What you will be ordering

The structural stage is steel beams, new floor joists with strutting and hangers, and floor boarding. Roof alterations add structural timber, tiles or cladding, membrane, battens and flashings. First fix consumes cable, back boxes, pipe and, for an en-suite, soil and waste fittings.

The finishing stages take rigid insulation boards for the rafters, plasterboard with screws, tape and jointing compound, finish plaster, acoustic and fire-rated boarding for the party wall where you share one, then the staircase itself, fire doors where required, floor finishes, skirting and paint. Order stage by stage: quantities firm up as the structure goes in, and the calculators above return the order quantities for each step.

Where the Building Regulations touch a loft conversion

Every calculator, by build stage